This is the last release before the announced license change (see [#2973](https://github.com/ufz/ogs/pull/2973) pull request), and we are moving to GitLab! GitHub has served us well in the past years but we would like to employ the more powerful feature set of GitLab. See the [announcement](https://discourse.opengeosys.org/t/development-will-be-moved-to-gitlab/529) for more details.
## Features
### Process' changes
- LiquidFlow move porosity/storage from MPL solid phase to medium scale. [#2872](https://github.com/ufz/ogs/pull/2872)
- Reactive transport process: enrich Phreeqc interface using direct memory access approach for equilibrium-controlled reactions. [#2883](https://github.com/ufz/ogs/pull/2883)
- Rename Groundwater flow to SteadyStateDiffusion. [#2860](https://github.com/ufz/ogs/pull/2860)
- ThermoHydroMechanics now accepts linear shape functions for displacement. [#2885](https://github.com/ufz/ogs/pull/2885)
- Output intgration point data in RichardsMechanics process for proper restart option. [#2911](https://github.com/ufz/ogs/pull/2911)
- RichardsMechanics now uses the alternative mass balance formulation. [#2914](https://github.com/ufz/ogs/pull/2914), [#2921](https://github.com/ufz/ogs/pull/2921)
- Computation of all secondary variables for output and restart in RichardsMechanics process is now consistent with the latest solution. [#2936](https://github.com/ufz/ogs/pull/2936)
- RichardsMechanics now relies on the solid's bulk modulus and Biot coefficient to compute grain compressibility. [#2955](https://github.com/ufz/ogs/pull/2955)
#### Numerics
- Add bulk element mappings for triangles. [#2901](https://github.com/ufz/ogs/pull/2901)
- Add mapping from points of a pyramid face to points in the pyramid element. [#2913](https://github.com/ufz/ogs/pull/2913)
- Remove unused time discretization schemes. Only backward Euler is being used now. This simplifies certain upcoming refactorings. [#2908](https://github.com/ufz/ogs/pull/2908)
#### Input and output
- Fixed bug of fixed time output not being written in case of repeated time step. [#3000](https://github.com/ufz/ogs/pull/3000)
- Added back names of properties for better error messages. [#2958](https://github.com/ufz/ogs/pull/2958)
- Store properties' definition scale for each property. [#2958](https://github.com/ufz/ogs/pull/2958)
- Improve error handling in MPL when accessing a property of wrong type. [#2941](https://github.com/ufz/ogs/pull/2941)
- Add computation of bulk modulus for MFront solid material models. [#2955](https://github.com/ufz/ogs/pull/2955)
- Add solid pressure. Used by RichardsMechanics for solid density update. [#2999](https://github.com/ufz/ogs/pull/2999)
- Generalize exponential property, which now has an additive offset. [#3008](https://github.com/ufz/ogs/pull/3008)
### File I/O
- Write PVD files for each subdomain output. [#2949](https://github.com/ufz/ogs/pull/2949).
### Testing and documentation
- Refactored FileTools.cpp to use std::filesystem. [#2918](https://github.com/ufz/ogs/pull/2918)
- Added documentation on how to use windows subsystem for linux (WSL). [#2946](https://github.com/ufz/ogs/pull/2946)
- Test definitions in the project files now support regular expressions. [#2945](https://github.com/ufz/ogs/pull/2945)
### New tools
- Tool to convert PVTU-meshes into regular, non-partishioned meshes and remove ghost information. [#2876](https://github.com/ufz/ogs/pull/2876)
- Add prism element type to structured mesh generator tool. [#2903](https://github.com/ufz/ogs/pull/2903)
- Create regular mesh consisting of pyramid elements. [#2909](https://github.com/ufz/ogs/pull/2909)
- Correct reordering for pyramid elements in NodeReorder tool. [#2937](https://github.com/ufz/ogs/pull/2937)
### Data Explorer
- Adding an array to a 2D mesh based on a raster file is now possible. [#2984](https://github.com/ufz/ogs/pull/2984)
## Infrastructure
- Serial container image is built with Python enabled. [#2765](https://github.com/ufz/ogs/pull/2765)
- Added CMake-function `ogs_add_library()` as a drop-in replacement for the `add_library()`-function. [#2881](https://github.com/ufz/ogs/pull/2881)
- Bumped minimum compiler and use `std::filesystem` for temporary file creation. [#2827](https://github.com/ufz/ogs/pull/2827)
- Replace `logog` logger with a modern and type safe [`spdlog`](https://github.com/gabime/spdlog) logger. It also includes the formatting library [`fmt`](https://fmt.dev/latest/index.html), which is a great alternative to the iostreams. [#2875](https://github.com/ufz/ogs/pull/2875), [#2873](https://github.com/ufz/ogs/pull/2873), [#2891](https://github.com/ufz/ogs/pull/2891), [#2882](https://github.com/ufz/ogs/pull/2882)
- Add GMRES restart to linear solver options. [#2910](https://github.com/ufz/ogs/pull/2910)
- More flexible algorithm to specify the file names of the output in the project file. [#2927](https://github.com/ufz/ogs/pull/2927)
- Now x_dot and dt are being passed to the `computeSecondaryVariables()` function allowing rate dependent secondary variable evaluation. [#2935](https://github.com/ufz/ogs/pull/2935)
- The order of `postTimeStep()` and `computeSecondaryVariables()` has been changed, s.t. all quantities computed in the secondary variables computation are available before the post time-step call. [#2936](https://github.com/ufz/ogs/pull/2936)
- Added GitLab CI config. [#2968](https://github.com/ufz/ogs/pull/2968)
### CMake options changes
- Removed option `OGS_USE_PCH`, added option `OGS_USE_UNITY_BUILDS` to speed up compilation by using [Unity builds](https://onqtam.com/programming/2019-12-20-pch-unity-cmake-3-16/). [#2906](https://github.com/ufz/ogs/pull/2906)
### Version info
New compiler minima ([#2827](https://github.com/ufz/ogs/pull/2827)):
- gcc: `9.0`, see also [#3006](https://github.com/ufz/ogs/pull/3006)
