add VecMatOnMeshLib toghether with tests
Replace mesh pointer with reference in MeshItems.
Explicitly specify access to base class.
Comment names of MeshitemDataPosition. Improve indentation.
Rename data_id to global_index.
Rename mesh_item_ID and key to ByLocation and LocationKey.
Rename MeshItem to Location.
Update documentation of Location.
Replace position triple with Location struct in MeshitemDataPosition.

Requires two less-then comparison operators.
Identation. Const reference.
Rename MeshItems to MeshSubset.
Rename ComponentDistribution to MeshSubsets.

This reflects the purpouse of this class better.
Remove redundant VectorComposition::_ndata equal to table size.
Remove unused VectorComposition::getNMeshes().
Remove unused VectorComposition::getNComponents().

Also remove _vec_comp_dis from the class.
Remove unused VectorComposition::_ordering_type.
Rename numberingByMeshItems(), remove numberingByComponent().
Rename OrderingType enum.
Specify type explicitly.
Rename VectorComposition to MeshDofMapping.