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