Avoids unused vmem.

conservativeResize() is not working as expected.

This trades a copy of nodes pointers vs. O(#Elements * #Nodes/2) complexity.

Update the constructors, getGlobalIndex(), and meshGetSubsets()
to include the variable_id.

The global component offsets for each variable are
stored in the _variable_component_offset.

Use dofSizeWith(out)Ghosts to be consistent with the LocalToGlobalIndexMap.

This should clarify the ambiguous meaning of this function used in parallel and
non-parallel environments.

MSS---the mesh subsets.

With unique_ptr the mesh_component_map must be created
before the unique_ptrs' content is moved.

MSS are the MeshSubsets.

This is just updating the interfaces and some of the for-loops
but will not compile until the using part is also updated.

This is used in the constructor.

This results in fastest (in number of read instructions) code.