Add interface to Tetgen's holes functionality

[acreegan]

Tetgen allows holes to be created in the tetrahedralized mesh through the tetgenio attribute holelist. Pytetgen does not currently provide an interface to this attribute.

This is a feature that is very difficult to work around for projects that require it, so implementing this would be a significant improvement to pytetgen.

[Nobregaigor]

I agree. I am also looking for this functionality as many of my meshes requires internal "empty" volumes. Do you think this could be a future implementation? Thanks!