Improve xarray metadata export

[jenshnielsen]

• Add units and long_name to match default xarray usage https://xarray.pydata.org/en/stable/getting-started-guide/quick-overview.html#attributes
• Ensure that the per array metadata is also exported to dataarrays (and not just datasets)
• Export parent dataset links
• Tests for all above
• Refactor logic to make the handling of data_vars attrs more clear

[codecov]

Codecov Report

Merging #3116 (9c3df20) into master (4b3fa55) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3116      +/-   ##
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+ Coverage   65.65%   65.67%   +0.01%     
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  Files         216      216              
  Lines       28750    28762      +12     
==========================================
+ Hits        18877    18889      +12     
  Misses       9873     9873              

[jenshnielsen]

@FarBo and @astafan8 ready for review