TetGen geometry import/export

Added functionality for reading and writing TetGen *.poly-files which contain the geometry for creating TetGen meshes. Also did some changes to the mesh reading methods.
The interface does not support the full extend of TetGens capabilities (e.g. boundary values or region attributes are currently not supported) because it is not straightforward how to extract/include such information out of/into out data structures.

It works with test examples and the actual file structure is simple enough. I'd suggest to do practical tests with @TomFischer and @nagelt once this is merged and to extend the interface if necessary.