[PL] GW: Add LocalAssemblerData skeleton.
[PL] GW: Fill init, assemble. Add shape matrices.
[AL] Add LocalDataInitializer.

This adds a mapping from element types to the
corresponding shape functions. The latter are
passed as a template parameter to the
LocalAssemblerData.
[PL] GW: Store hydraulic conductivity in each elem.

This shall be changed later to incorporate different
material properties.
[PL] GW: Rename LocalAssemblerDataBase to Interface.

Same change in the AssemblerLib for consistency.
[PL] GW: Shorten shape_matrix names.