prospector-beta priors

[bd-j]

Thanks @wangbingjie

I think the fast priors are probably, fine, but he tests are failing with the folowing error I think in the template

E     File "/home/runner/work/prospector/prospector/prospect/models/templates.py", line 712
[43](https://github.com/bd-j/prospector/pull/261/checks#step:6:44)
E       'depends_on': transforms.nzsfh_to_zred}]
[44](https://github.com/bd-j/prospector/pull/261/checks#step:6:45)
E                                              ^
[45](https://github.com/bd-j/prospector/pull/261/checks#step:6:46)
E   SyntaxError: unmatched ']'

[bd-j]

Also, I think it might be helpful to be a bit more modular in the priors. Can you put the priors that require datafiles -- basically all of the ones from prosp_beta_priors -- into a separate module, maybe prospect/models/priors_beta.py but keep the priors from fast_distributions.py in the priors module (basically as in the first commit in this PR)

[bd-j]

We could also put all the commits after the first one in a separate PR - the first commit passed the tests and we could merge it now!

[bd-j]

It might be nice to add a description of the prospector beta priors and link to the paper in the docs. I can do that later or you can add as part of this PR.