Expand common_mean_bayes() to include test for differing precision

[Swanson-Hysell]

I just committed a common_mean_bayes() function to ipmag.py that implements the Heslop and Roberts (2018) common mean/reversal test.

I have utilized their code to implement the version of the test that us for populations with common precision (Appendix A; jgrb52994-sup-0003-Data S2.unk).

We should also implement the version with differing precisions (could be a flag in the same function), but this involves import of a special library quadpy which we may not want to make a dependency of ipmag.py. The version of the test is documented in Appendix B and SI materials: jgrb52994-sup-0004-Data S3.unk.

[Swanson-Hysell]

Putting this here as a feature request issue as a reminder to myself to work on this down the line.