Prevent merging of different variables with the same name and date but different attributes

[Metamess]

closes #336

Fixes a bug where cfgrib would incorrectly combine two different variables if they happened to have the same shortName and data. For example, all-zeroes data GRIB messages with stepType='instant' and stepType='avg' attributes for csrwe (Convective snowfall rate water equivalent) on days without snow.

[FussyDuck]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (3951355) 95.44% compared to head (597b2ff) 95.44%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #339   +/-   ##
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  Coverage   95.44%   95.44%           
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  Files          26       26           
  Lines        2040     2040           
  Branches      236      236           
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  Hits         1947     1947           
  Misses         62       62           
  Partials       31       31           

Impacted Files	Coverage Δ
cfgrib/xarray_store.py	93.84% <100.00%> (ø)

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[iainrussell]

Hi @Metamess,

Thank you for providing this great fix! I think the xarray documentation is not great when explaining exactly how all this matching works, but I could get the relevant data, reproduce the bug and confirm that your fix works. I just added a couple of tests using very small data, and fixed the 'black' formatting (you may have noticed that the 'code style' test failed. Hopefully it will go through now, and then I will merge it in. I also appreciated your comprehensive description of the problem - very helpful!
Cheers,
Iain