Add units, input Nones for missing variable attrs

[dougiesquire]

This PR adds variable units attributes to the datastores and sets variable attributes to None when they are not available. Thus, the attributes for each variable can be determined from their index in the list.

cc @aidanheerdegen, @utkarshgupta95

Closes #120

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (b891b5c) 97.31% compared to head (f91a2aa) 97.16%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
- Coverage   97.31%   97.16%   -0.15%     
==========================================
  Files           9        9              
  Lines         559      565       +6     
==========================================
+ Hits          544      549       +5     
- Misses         15       16       +1     

Files	Coverage Δ
src/access_nri_intake/source/__init__.py	100.00% <100.00%> (ø)
src/access_nri_intake/source/builders.py	93.61% <ø> (ø)
src/access_nri_intake/source/utils.py	97.84% <83.33%> (-0.66%)	⬇️

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