Support sweeping a face of an existing shell

[hannobraun]

Right now, it is possible to sweep a Face along a path, creating a Shell. This is useful for creating a new shell.

It would also be useful to extend an existing shell, by selecting one of its faces and sweeping that, creating new objects that attach to the shell. This would be very similar to the existing operation, except that the new objects should not attach the the original face, but to its neighbors, and the original face should be removed from the shell. Thus, it should be possible to reuse most of the existing code, but some refactoring will certainly be needed.

I am going to start working on this now.

[hannobraun]

Okay, so I was really bad about keeping this issue updated with my progress, but it's finished now 😄

It's now possible to extend a shell by sweeping one of its faces. This is a screenshot from the split model.

The API to do it is still pretty clunky, but it's a step in the right direction.