- 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)

This reverts commit 05677165.

Conflicts:
	FileIO/XmlIO/XmlGspInterface.cpp
	README.md

- renamed method to access coordinates from getData() to getCoords()

changed size_t to unsigned; included a few (yet unimplemented) methods for mesh-consistency checking

- fixed some warnings regarding the sequence of attributes in header file and initializer list in constructor

added some basic functionality (get, add, etc.) and introduced an example finite element class for tetrahedra