Maximum angle constraints

[DanielVandH]

Mesh refinement in this package does currently have a max_angle option, it just errors if you try to use it. I did try and do it, but ran out of time. I do already have full implementations of triangle sinks and triangle offcenters though, but more is needed. It would be nice to implement the very nice algorithms from https://doi.org/10.1109/ISVD.2009.32 and https://doi.org/10.1137/080716748 (starting by implementing the second, which is not about maximum angles but about better Steiner points, is probably optimal for understanding the maximum angle paper).

http://cccg.ca/proceedings/2007/09a2.pdf