[BL] Use std::chrono::system_clock for wall-clock.

RemoveGhostData cmd line utility

[CMake] Fix segmentation fault in ThirdParty/IPhreeqc

Categorized the files of relative permeability and capillary pressure models

Rename Groundwater flow to SteadyStateDiffusion

- test data folders,
- tests names, and
- process names in the project files.

- Rename documentation files and add docu.

Rename the process type from GROUNDWATER_FLOW to STEADY_STATE_DIFFUSION.

Rename the tag for the process in project files.

Rename namespaces and classes.

Rename files and change includes.
Renamed:
- GroundwaterFlowProcess.{h,cpp} to SteadyStateDiffusion.{h,cpp}.
- GroundwaterFlowProcessData.h to SteadyStateDiffusion.h.
- GroundwaterFlowFEM.h to SteadyStateDiffusionFEM.h.

Rename:
- CreateGroundwaterFlowProcess to CreateSteadyStateDiffusion.

- Rename
  + process name,
  + process folder, and
  + cmake macros.

Check headers

The c++ standard flags are already set.

spdlog cleanups