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.
Yes. These are explicitly added to show the future usage.
Please refer to the commit https://github.com/endJunction/ogs/commit/b14de0b21525d1cac417f564aa056dfcb93b638c.