Implement checking of domain variables against cell_measures

[benjwadams]

Implementation of 5.8 Domain Variables when checking against cell measures, conformance required 4/4 in CF 1.9 standard.

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 81.65%. Comparing base (b3f1891) to head (e3486ad).
Report is 6 commits behind head on develop.

Files	Patch %	Lines
compliance_checker/cf/cf_1_9.py	70.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1089      +/-   ##
===========================================
- Coverage    81.68%   81.65%   -0.04%     
===========================================
  Files           25       25              
  Lines         5226     5227       +1     
  Branches      1255     1257       +2     
===========================================
- Hits          4269     4268       -1     
- Misses         646      648       +2     
  Partials       311      311              

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