Proposal A: Derived mesh class for domain decomposition approach (Do not merge)

To  @norihiro-w  @rinkk @TomFischer :

This is a proposal to handle additional mesh data for  domain decomposition approach. This idea is to encapsulate all related data in a derived class of class mesh. By this way, ``#ifdef``  is needed in local assembly for  MPI compilation. Of coarse, I can use template argument to avoid ``#ifdef`` if  the local assembly is finished.  Its usage would be
```
#ifdef USE_MPI
	MeshLib::MeshDDC* mesh = FileIO::readMeshFromFile(fname);
#endif
```
On the contrary, class Element can be also derived with additional data member for active node indices, this is demonstrated in another  PR (#421). With later local  ``#ifdef`` is avoid, but an index mapping to access element nodes is required.

Please give suggestion to me about Proposal A and B (#421), and any other idea about this issue.