Assembler lib

These changes in short separate the global assembler (`VectorMatrixAssembler`) from the local assembler types (almost) and add a possibility to provide a specific local assembler for each element or element type.
A simple usage is shown in the tests (last commit), though very simplistic.

I'm open for name suggestion for the `SimpleAssembler`.
I tried to address this issue by renaming this class to LocalAssemblerBuilder, although I'm not very happy with the new name too (which is better now anyway).