Bugfix: add flat powerlaw to gp_priors.py

[jeremy-baier]

enterprise_extensions has a flat powerlaw model in the common_red_noise_block which tries to use a GP prior from enterprise, but it does not exist in the main or dev branches. This PR adds that GP prior.

line in enterprise extensions which calls the flat powerlaw:
https://github.com/nanograv/enterprise_extensions/blob/4e1ce072f179ceb572f593e7af15329c3f645d77/enterprise_extensions/blocks.py#L1460

flat powerlaw taken from:
https://gitlab.in2p3.fr/epta/enterprise/-/blob/master/enterprise/signals/gp_priors.py?ref_type=heads

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.61%. Comparing base (e04d9d4) to head (3a6da88).
⚠️ Report is 27 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #423      +/-   ##
==========================================
- Coverage   72.53%   71.61%   -0.93%     
==========================================
  Files          13       13              
  Lines        3401     3252     -149     
==========================================
- Hits         2467     2329     -138     
+ Misses        934      923      -11     

Files with missing lines	Coverage Δ
enterprise/signals/gp_priors.py	89.70% <100.00%> (+5.42%)	⬆️

... and 5 files with indirect coverage changes

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[AaronDJohnson]

Looks good to me. In the future, we should consider maintainability improvements by creating a base powerlaw function that takes df as an argument. It would reduce code duplication and keep speed the same.