Refactor dof table construction

Mainly moving createDofTable() function into LocalToGlobalIndexMap constructor.
This removes duplicate code from the GroundwaterFlowProcess and the SteadyDiffusion2D test.

Other important changes are of technical nature:
 - Iterator access for mesh's elements
 - Disable implementation of LocalToGlobalIndexMap for parallel environments. This was not implemented or tested anyway.
The size function was not used in Release mode, but it is used in an assertion in [VectorMatrixAssembler](https://github.com/ufz/ogs/blob/master/AssemblerLib/VectorMatrixAssembler.h#L50).
So, I've put it back.