integrated new mesh-classes into gui, rewrote some methods because everything is much shorter and easier now; some fem-related files and adaptions are still missing

Commit ff14379f243ac66935384a56407fadcee6d23cce from OGS-5.

- optimized addNeighbor()

[Mesh]
- optimized setNeighborInformationForElements()

Conflicts:
	MeshLib/Elements/Edge.cpp

- added (a not yet finished version) CollapseMeshNodes programme
- changes in SimpleTests/MeshTests/CMakeLists.txt

[MeshCoarsener]
- new class used by CollapseMeshNodes
- can be used to make mesh nodes unique within a small epsilon radius

[MeshIO]
- added writing functionality for old mesh format

[GeoLib::Grid]
- changed interface (method getPointsWithinCube() -> getVecsOfGridCellsIntersectingCube())
- corrected documentation

[all mesh elements]
- added method clone()
- added method getType()
- removed attribute type from abstract base class Element