Add cython versions of geometric calculations

This MR addresses #576 (closed).

Two other methods have been proposed !968 (closed) and !975 (closed).

• !968 (closed) gets a speedup of ~1.5x by combining two calculations.
• !975 (closed) gets a speedup of ~10x at the cost of tying us to lal functions. This necessarily loses some flexibility.

In this MR I add an additional extra little package I wrote with cython versions of some functions. This gets both of the advantages of the other MRs. See the README of that project for more details on how reliable/much faster this is.

I also changed a few things, the biggest practical speedup is in the change of frame from detector-based to geocentric coordinates that also avoids the ugly caching.

Another option would be to add the cython directly into Bilby. I guess we can't avoid having the cython dependency if we use this package.

If people like this approach, I will start the process of properly packaging the cython so we can merge this.