ENH: calculate solid angles of Voronoi cell faces

[pmla]

In this PR I have implemented solid angle calculation for Voronoi cell faces.

Quite a few downstream projects use Voro++ to calculate these. It would be very convenient and save a lot of duplication to have this functionality in Voro++ itself.

[chr1shr]

Thanks for adding this functionality. I have approved this merge.

We are currently working on a development version of the library, and I will ensure the solid_angles function is added to this version although I may revise the implementation then. Two small comments:

• There are some possibilities for numerically undefined behavior, such as when norm is zero. There are reasonable situations where that could happen, such as when walls have cut a Voronoi cell such that one of the vertices lies at the particle position. Perhaps solid angles aren't relevant but it might be good for the routine to have some way to handle this.
• Some of the dot products will be computed multiple times for adjacent triangles making up a polygonal face and they could potentially be re-used in the computation.