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`.
for me `LocalInitializer` sounds better because it initializes only local data.