Check only if there is a neighbor.

This is used when the face_id of the neigbor is known.

added functionality to find all nodes that are connected via edges and via element to other nodes; replaces all cout's with appropriate logog messages

fixed gmshinterface; fixed mesh copy constructor; fixed layer mapping; removed various TODO6 tags. DIRECT STs still not working; FEFLOW and TETGEN still not working; getsurface still not working

this leads to the other changes of this commit in the inheritance hierarchy
introduced a const_cast in Element::addNeighbor()

- optimized addNeighbor()

[Mesh]
- optimized setNeighborInformationForElements()

- 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

added first version to find and store neighbours of all elements during mesh initialisation (work in progress)