Merge pull request #2823 from endJunction/RichardsMechanicsBishopsEffectiveStress

.../properties/property/BishopsPowerLaw/c_BishopsPowerLaw.md

...ectFile/properties/property/BishopsPowerLaw/t_exponent.md

...erty/BishopsSaturationCutoff/c_BishopsSaturationCutoff.md

...erties/property/BishopsSaturationCutoff/t_cutoff_value.md

MaterialLib/MPL/CreateProperty.cpp

MaterialLib/MPL/Properties/BishopsPowerLaw.cpp

MaterialLib/MPL/Properties/BishopsPowerLaw.h

MaterialLib/MPL/Properties/BishopsSaturationCutoff.cpp

MaterialLib/MPL/Properties/BishopsSaturationCutoff.h

MaterialLib/MPL/Properties/CreateBishopsPowerLaw.cpp

MaterialLib/MPL/Properties/CreateBishopsPowerLaw.h

MaterialLib/MPL/Properties/CreateBishopsSaturationCutoff.cpp

MaterialLib/MPL/Properties/CreateBishopsSaturationCutoff.h

MaterialLib/MPL/Properties/CreateProperties.h

MaterialLib/MPL/Properties/PorosityFromMassBalance.cpp

MaterialLib/MPL/Properties/Properties.h

MaterialLib/MPL/PropertyType.h

MaterialLib/MPL/VariableType.h

ProcessLib/RichardsMechanics/RichardsMechanicsFEM-impl.h

ProcessLib/RichardsMechanics/Tests.cmake