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.
Had to try myself. It would return a reference to local object.
clang says:
`non-const lvalue reference to type 'RowColumnIndices<[...]>'
cannot bind to a temporary of type 'RowColumnIndices<[...]>'`