Flag fixes

[benjwadams]

Fixes to flags:

• Checks that 0 does not appear in flag_masks
• Fixes flag_values checking when attribute is single-valued/scalar (fixes Update check_cell_measures() function in cf_1_6.y #992)

[codecov]

Codecov Report

Merging #924 (dfd8b34) into master (f132cc3) will increase coverage by 0.13%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #924      +/-   ##
==========================================
+ Coverage   78.58%   78.72%   +0.13%     
==========================================
  Files          23       23              
  Lines        5259     5264       +5     
  Branches     1307     1307              
==========================================
+ Hits         4133     4144      +11     
+ Misses        834      829       -5     
+ Partials      292      291       -1     

Impacted Files	Coverage Δ
compliance_checker/cf/cf_1_6.py	88.75% <100.00%> (+0.63%)	⬆️

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[benjwadams]

@pp-mo, I can't add you as a reviewer presently, but please see if this fixes issues you were having with #992.

[pp-mo]

see if this fixes issues you were having with #992.

Thanks, yes it does.
Closed #924 in anticipation.