[A] Remove unnecessary virtual function specifier.
[A] Document ProjectData class.
[A] Correct ProjectData function names. Correct constness.
[A] Add mesh check in ProjectData::parseProcessVariables.

Also clarify an error message and the TODO for meshes.
[A] Move unused (from outside) methods into private sec.
[A] Use non-static member init for ProjectData::geoObjects.

This removes ProjectData ctor;
[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] Use number of node's DOF in createDOFMap().
[PL] Remove MeshComponentMap include from GW.
[PL] Add dof_map to GroundwaterFlow.
[PL] Empty solve().
[PL] Add empty GroundWaterflowFEM.