Enable handing of multi-region meshes and meshes containing holes

[Kevin-Mattheus-Moerman]

This requires proper handling of the: boundary marker, hole point, and region point attributes.

[Kevin-Mattheus-Moerman]

For a MATLAB wrapper see the GIBBON project and the file:
https://github.com/gibbonCode/GIBBON/blob/master/docs/HELP_runTetGen.m

[SimonDanisch]

This should work now :) We should add some docs though!