Empty GroundwaterFlowProcess and its Base classes.
[PL] Add ProcessLib to CMake files.
Add ProcessLib to doxygen inputs.
[PL] Add virtual initialize().
[PL] Add number of node variables.

Element variables are not yet used.
[PL] Add mesh subset to initialize().
[PL] Initialize MeshSubsets and MeshComponentMap.
[PL] Rename mesh_items_all_nodes; items->subset.
[PL] Add destructor for GroundwaterFlowProcess.
[PL] Move mesh_component_map to class variables.

...from the initialize().
Also delete it in the dtor.
[PL] Add createDofMap() in Process.
[PL] Remove MeshComponentMap include from GW.
[PL] Add dof_map to GroundwaterFlow.