Add ability in ENIGMA calibration script to fix a subset of calibration coefficients

[prayush]

Currently, this line assumes all coefficients are being sampled over:

gwnrtools/gwnrtools/stats/enigma_utils.py

Line 350 in d5d4822

coeffs = theta[1:]

Instead, one must check the all_params DataFrame for a list of parameters held constant and use those too. To do so, one needs the ordered sampled params, and another ordered list of all params used in a particular fit.