IDL comparison tests for contribution functions for various ions

[wtbarnes]

This PR adds several tests that compare the output from fiasco and CHIANTI IDL for the contribution functions for several ions. The IDL results are also saved and included here, with the intention of including comparison plots in the documentation.

The IDL tests are now run on the CI using the cached results. Additionally, the hash checking for the files has been refactored a bit and a hash generation function has been added as well as a JSON files with all hashes for the current version of the database.

Also fixes #256

[codecov-commenter]

Codecov Report

Attention: 15 lines in your changes are missing coverage. Please review.

Comparison is base (2d8e308) 90.56% compared to head (f0989ef) 91.90%.

Files	Patch %	Lines
fiasco/tests/idl/helpers.py	70.96%	9 Missing ⚠️
fiasco/conftest.py	84.61%	2 Missing ⚠️
fiasco/tests/idl/conftest.py	50.00%	2 Missing ⚠️
fiasco/tests/idl/test_idl_continuum.py	93.75%	1 Missing ⚠️
fiasco/util/setup_db.py	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #257      +/-   ##
==========================================
+ Coverage   90.56%   91.90%   +1.34%     
==========================================
  Files          36       38       +2     
  Lines        2670     2718      +48     
==========================================
+ Hits         2418     2498      +80     
+ Misses        252      220      -32     

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