Addition of new cooling curves

[jordidj]

PR description

Addition of the colgan_dm cooling curve and its constituent curves colgan and dalgarno2

New features

Legolas

• Three new cooling curves: colgan, dalgarno2, colgan_dm

Bugfixes

Unit tests

• Missing initialisation statement in spex_dalgarno_below

Checklist

• all tests pass (and for new features, new tests are added)
• documentation has been updated (if applicable)

[codecov]

Codecov Report

Merging #137 (77b5c08) into develop (6a2f6da) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #137      +/-   ##
===========================================
+ Coverage    90.68%   90.69%   +0.01%     
===========================================
  Files          158      158              
  Lines         8952     8965      +13     
===========================================
+ Hits          8118     8131      +13     
  Misses         834      834              

Flag	Coverage Δ
legolas	93.13% <100.00%> (+0.01%)	⬆️
pylbo	86.33% <ø> (ø)

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