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.
looks redundant having getElements() and these functions. one can get iterators using a result of getElements()