Prism gravity consistent around the prism

The gravitational fields of a prism should be consistent around it. There was a problem with the arctan2 quadrants and log(0)s. There is also a problem with the tensor cross-components when the computations point is aligned with one of the edges of the prism on a certain face. This last one was fixed by moving the point slightly when it happens. This doesn't give the best results and I had to reduce the accuracy of the tests for this.

Checklist:
- [x] Create/update docstrings
- [x] Make tests for new code
- [x] All tests pass
- [x] Can be merged
- [x] Changelog entry