Readability: mat_values -> material_values.

mat_values can be confused with matrix values, instead mat_values stand for material values.
Removed redundant typedef.
[LocalGWAssemblerData] Material data as template.
[LocalGWAssemblerData] Impl. of get and set for material.

If the material data type is a pointer the get method does not return a pointer to the data. The value of data type is returned.
Using the type of the property of LocalGWAssemblerData.