refactor load_cmipng_tas and load_cmipng_hfds

[mathause]

This adds a common function for load_cmipng_tas and load_cmipng_hfds as suggested by your comment. The only difference was the use of np.average with a masked vs not-masked array. As tas has data everywhere this should not make a difference.

Next up are then tests and then an internal refactor.

[mathause]

I might still want to add tests here (after #52)

[codecov-commenter]

Codecov Report

Merging #55 (61305b3) into master (d73e8f5) will increase coverage by 0.81%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master      #55      +/-   ##
==========================================
+ Coverage   77.53%   78.34%   +0.81%     
==========================================
  Files          26       26              
  Lines        1322     1293      -29     
==========================================
- Hits         1025     1013      -12     
+ Misses        297      280      -17     

Flag	Coverage Δ
unittests	78.34% <89.47%> (+0.81%)	⬆️

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Impacted Files	Coverage Δ
mesmer/io/_load_cmipng.py	79.36% <88.88%> (ø)
mesmer/io/__init__.py	100.00% <100.00%> (ø)

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[mathause]

This would be ready for review. More refactoring could be done but let's do this in another PR.