renamed struct for type of element from FEMElemType to CellType and renamed virtual method getFEMType() to getCellType() in Elemnts class hierarchy

added (1) FEMElemType to elements as template parameter, (2) method getFEMType() and (3) an enum for FEM element types

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