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`.
why not pass `ex1._localA` and  `ex1._localRhs` to `LocalDataInitializer`? it'd be better if problem specific stuff are put together.