Add a function IsDigraphRotationSystem #869

The function FacialWalks takes something called a “rotation system” as its second argument.

I'm not exactly sure what this is, but anyway, FacialWalks does an incomplete check that its second argument is a rotation system. I thought that as well as making this check more robust/accurate, it would be good to have it as a standalone function.

In other words, we should implement IsDigraphRotationSystem, and FacialWalks should use it to validate its arguments.