Create dof map

This adds a construction of DOF map for processes. In particular it is used in the GroundwaterFlowProcess.

The first two commits are adding the createDofMap() to ProcessLib::Process.
The future usage of that function is implemented in the following three commits.
__Update:__ the github is rearranging the commits: the above sets are: [071b75e, 5f4d490] and [50f154b, 5e99639,  b14de0b].
Nice method!