Merge pull request #2375 from TomFischer/MPL

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

...arProperty/independent_variable/i_independent_variable.md

...earProperty/independent_variable/t_reference_condition.md

...s/property/LinearProperty/independent_variable/t_slope.md

...ty/LinearProperty/independent_variable/t_variable_name.md

...e/properties/property/LinearProperty/t_reference_value.md

MaterialLib/MPL/Component.h

MaterialLib/MPL/CreateProperty.cpp

MaterialLib/MPL/Medium.h

MaterialLib/MPL/Properties/LinearProperty.cpp

MaterialLib/MPL/Properties/LinearProperty.h

MaterialLib/MPL/Property.cpp

MaterialLib/MPL/Property.h

MaterialLib/MPL/VariableType.cpp

MaterialLib/MPL/VariableType.h

Tests/MaterialLib/TestMPLLinearProperty.cpp