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].