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

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

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