[Docu] Docu for the MPL/LinearProperty file tags.

Documentation/ProjectFile/properties/property/LinearProperty/c_LinearProperty.md

+Definition of a linear property:
+\f$y(x) = y_{\textrm{ref}} + m (x - x_{\textrm{ref}})\f$
+where
+ - \f$y_{\textrm{ref}}\f$ is a reference value, for instance reference density
+ - \f$m\f$ is the slope of the linear relationship
+ - \f$x_{\textrm{ref}}\f$ is a reference condition, for instance reference
+   temperatue

Documentation/ProjectFile/properties/property/LinearProperty/independent_variable/i_independent_variable.md

+A linear constitutive property.

Documentation/ProjectFile/properties/property/LinearProperty/independent_variable/t_reference_condition.md

+A numeric value for the reference condition \f$x_{\textrm{ref}}\f$ as one of
+the implemented data types given in MaterialPropertyLib::PropertyDataType.

Documentation/ProjectFile/properties/property/LinearProperty/independent_variable/t_slope.md

+The slope \f$m\f$ of the linear relation.

Documentation/ProjectFile/properties/property/LinearProperty/independent_variable/t_variable_name.md

+The variable type the linear relation depends on.

Documentation/ProjectFile/properties/property/LinearProperty/t_reference_value.md

+A numeric value for the reference value \f$y_{\textrm{ref}}\f$ as one of
+the implemented data types given in MaterialPropertyLib::PropertyDataType.