[TRM] Tests for a bentonite material model (implemented in generalmod, used via MFront)

• Adds five test cases for a bentonite material model:
  • 0D confined compression
  • 0D saturation
  • 1D column saturation
  • 1D column saturation restart (success and failure)
• Adds the tested bentonite material model (only usable for testing, not shipped with OGS binaries)
  • license: GPL2 or later
• Adds some Python scripts for
  • batch plots to visualize 0D and 1D test results
  • generating reference vtu files from MTest results
  • scripts to be included in OGSTools in the future
• Makes saturation an optional material property in TRM (to not reset it upon restart)

1. Feature description was added to the changelog
2. Tests covering your feature were added?
3. Any new feature or behaviour change was documented?