Add calculations for angles between edges and angles between faces

[macumber]

For a face, should be able to calculate the interior and exterior angles between two edges joined by a vertex. Exterior angles should sum to 360 degrees. Interior angles should sum to (n_sides - 2)*180 degrees.

For a cell, should be able to calculate the interior and exterior angles between two faces joined by an edge.

See https://github.com/rd2/tbd/blob/6ee3d7b106f8e6f8d89df7097549baab61be2572/lib/tbd/psi.rb#L1086 for use case and possible implementation