This reverts commit 14cdf0c6.

Needed for choosing integration method at compile time.

- 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 some small helper functions (face normal calculation, on-surface-check, etc.), renamed MshElemType::LINE to EDGE, changed a bit of documentation

This reverts commit 05677165.

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

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