Add inspection testing covering any construct lacking data

[sadielbartholomew]

Follow-on PR to #175 to add some extra testing covering all other constructs that weren't buggy but it would regardless be good to check in a similar way, namely to call str() and repr() on when the constructs have no data, for any which can take data, so excluding only cell methods and coordinate references from the data model. Context can be found in #175 (review).

@davidhassell shall I also add equivalent tests to cf-python, or is that overkill?

[codecov]

Codecov Report

Merging #176 (d7fc3b9) into master (76d2102) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #176      +/-   ##
==========================================
+ Coverage   88.71%   88.74%   +0.03%     
==========================================
  Files         101      101              
  Lines       10806    10806              
==========================================
+ Hits         9586     9589       +3     
+ Misses       1220     1217       -3     

Flag	Coverage Δ
unittests	88.74% <ø> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cfdm/read_write/netcdf/netcdfread.py	83.50% <0.00%> (+0.06%)	⬆️
cfdm/field.py	87.06% <0.00%> (+0.20%)	⬆️
cfdm/domain.py	95.68% <0.00%> (+0.49%)	⬆️

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[davidhassell]

Hi Sadie - nice generalised tests. All good to go, I think.