Merge pull request #2649 from renchao-lu/EnrichLinearProperty

...le/properties/property/LinearProperty/c_LinearProperty.md

...Property/independent_variables/i_independent_variables.md

..._variables/independent_variable/i_independent_variable.md

...t_variables/independent_variable/t_reference_condition.md

...rty/independent_variables/independent_variable/t_slope.md

...pendent_variables/independent_variable/t_variable_name.md

MaterialLib/MPL/CreateProperty.cpp

MaterialLib/MPL/Properties/LinearProperty.cpp

MaterialLib/MPL/Properties/LinearProperty.h

...ata/Parabolic/HT/FaultedCube/Ra_795_fault_bcgs_jacobi.prj

.../Data/Parabolic/HT/SimpleSynthetics/constraint_bc_1e3.prj

...redCoupling/ADecovalexTHMCBasedHTExample/th_decovalex.prj

...ling/ConstViscosity/square_5500x5500_staggered_scheme.prj

...scosity/square_5500x5500_staggered_scheme_adaptive_dt.prj

...droMechanics/Linear/Beam_sealed_bimaterial/square_1e2.prj

...oMechanics/Linear/Beam_unsealed_bimaterial/square_1e2.prj

...Mechanics/Linear/Square_sealed_homogeneous/square_1e0.prj

Tests/MaterialLib/TestMPLLinearProperty.cpp