[web] Added page on Snakemake workflow usage.

web/content/docs/devguide/testing/workflow-testing.md

++++
+date = "2021-03-05T10:49"
+title = "Workflow testing"
+author = "Lars Bilke"
+weight = 1024
+
+[menu]
+  [menu.devguide]
+    parent = "testing"
++++
+
+## Introdution
+
+We use the workflow manager [Snakemake](https://snakemake.readthedocs.io) to test workflows which consist of the execution of several steps which are based on each other.
+
+## Requirements
+
+- [Snakemake](https://snakemake.readthedocs.io/en/stable/getting_started/installation.html) >= {{< dataFile "versions.minimum_version.snakemake" >}}
+    - If you use [Poetry]({{< ref "python-env.md#poetry" >}}) then `snakemake` is installed in your virtual environment in your build-directory automatically. You can then call it via `poetry run snakemake ...`.
+- On Windows only:
+    - The `tee`-utility in the `PATH` (can be installed from https://sourceforge.net/projects/unxutils)
+
+## Examples
+
+- [ExtractBoundary.smk](https://gitlab.opengeosys.org/ogs/ogs/-/blob/master/Applications/Utils/ExtractBoundary.smk)
+- [VoxelGridFromLayers.smk](https://gitlab.opengeosys.org/ogs/ogs/-/blob/master/Applications/Utils/VoxelGridFromLayers.smk)
+
+These example workflows [are added to ctest](https://gitlab.opengeosys.org/ogs/ogs/-/blob/540d0b454c9e3805a81f7c4a1b6ee7565be6845c/Applications/Utils/Tests.cmake#L302-315) as well:
+
+```cmake
+if(SNAKEMAKE AND NOT OGS_USE_MPI)
+    add_test(NAME snakemake_ExtractBoundary
+        COMMAND ${SNAKEMAKE} -j 1
+            --configfile ${PROJECT_BINARY_DIR}/buildinfo.yaml
+            -s ${CMAKE_CURRENT_SOURCE_DIR}/ExtractBoundary.smk
+    )
+    add_test(NAME snakemake_VoxelGridFromLayers
+    # ...
+    )
+    add_dependencies(ctest ExtractBoundary Layers2Grid AddFaultToVoxelGrid)
+endif()
+```
+
+## Modularization
+
+We started on implementing modular rule definitions and tool wrapper in [`scripts/snakemake`](https://gitlab.opengeosys.org/ogs/ogs/-/tree/master/scripts/snakemake).
+
+## Links
+
+- [Snakemake Documentation](https://snakemake.readthedocs.io)
+- [Short tutorial](https://snakemake.readthedocs.io/en/stable/tutorial/short.html)
+- [Software Carpentry Workshop](https://carpentries-incubator.github.io/workflows-snakemake/index.html)
+- [HPC Carpentry Workshop with Snakemake](http://www.hpc-carpentry.org/hpc-python/11-snakemake-intro/index.html)