Splitattrs ncsave redo commonmeta

[pp-mo]

MISTAKE - replaced by pp-mo#77

just testing
provisional changes to common-metadata to accommodate split attribute dictionaries

No new function yet -- just to see if it breaks any existing tests
Will need additional tests for how this actually functions with split-attr testcases

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Consult Iris pull request check list

[codecov]

Codecov Report

Patch coverage: 70.00% and project coverage change: -0.12% ⚠️

Comparison is base (68eaa53) 89.41% compared to head (7abf840) 89.30%.

Additional details and impacted files

@@                   Coverage Diff                   @@
##           FEATURE_split_attrs    #5420      +/-   ##
=======================================================
- Coverage                89.41%   89.30%   -0.12%     
=======================================================
  Files                       89       89              
  Lines                    22500    22595      +95     
  Branches                  5396     5423      +27     
=======================================================
+ Hits                     20119    20179      +60     
- Misses                    1636     1667      +31     
- Partials                   745      749       +4     

Files Changed	Coverage Δ
lib/iris/common/metadata.py	88.95% <36.53%> (-4.29%)	⬇️
lib/iris/__init__.py	88.97% <50.00%> (+0.08%)	⬆️
lib/iris/fileformats/netcdf/saver.py	89.48% <93.84%> (+0.22%)	⬆️
lib/iris/cube.py	90.98% <100.00%> (+0.01%)	⬆️

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