Fix RMSF and add return vals

[AlecThomson]

Closes #128

[AlecThomson]

Python 3.8 tests seem to be failing on Bilby issues for some reason. But otherwise I think this is looking ok

[AlecThomson]

Interesting, numpy-1.22.3 and bilby-2.2.2 seem to be incompatible on the Python 3.8 tests. Bumping the numpy version up in the tests seemed to do the trick. I'll try to put this in the setup.py to avoid the issue for users

[AlecThomson]

@Cameron-Van-Eck when you get a chance to squiz, I think this is g2g

[Cameron-Van-Eck]

I feel like I haven't entirely been sold on these NamedTuples, but I can't deny that they solve the problem in this case. I should really spend some time getting comfortable with them...