Add Itoh integrated Gaunt factors to f-f radiative loss

[jwreep]

Fixes #248

• Parse the new files
• Add Itoh options to total f-f Gaunt factor
• Add Itoh options to f-f radiative loss function
• Tests

[jwreep]

For my own reference, the equations from Itoh et al 2002 for the two forms of the Gaunt factor. (They're using log base 10.)

Relativistic:

Non-relativistic:

[wtbarnes]

One request before this PR gets too far along. Would you mind making this PR from a branch on your fork other than main? In cases where I need to check out a PR locally, things get a bit confused when the PR branch is also called main. My usual strategy is to create a new branch for every PR.

You should just be able to move all of these commits directly from this branch to a new one and then open a PR from that branch.

[jwreep]

Yep, will do. Sorry about that!

[codecov]

Codecov Report

Attention: Patch coverage is 72.72727% with 6 lines in your changes missing coverage. Please review.

Project coverage is 93.00%. Comparing base (8ee514b) to head (92853a7).

Files	Patch %	Lines
fiasco/io/sources/continuum_sources.py	72.72%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #292      +/-   ##
==========================================
- Coverage   93.14%   93.00%   -0.15%     
==========================================
  Files          37       37              
  Lines        3079     3101      +22     
==========================================
+ Hits         2868     2884      +16     
- Misses        211      217       +6     

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