feat: Add override to UserMetadata

[HCookie]

Update UserMetadata

• Add override method
  • Complete deep copy
• Add dump

Allows

from earthkit.data.utils.metadata.dict import UserMetadata

metadata = UserMetadata({'shortName':'2t'})
metadata.override(shortName = 'msl')

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.01%. Comparing base (c80c8db) to head (64ec255).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #683      +/-   ##
===========================================
+ Coverage    90.99%   91.01%   +0.01%     
===========================================
  Files          162      162              
  Lines        12363    12385      +22     
  Branches       605      609       +4     
===========================================
+ Hits         11250    11272      +22     
  Misses         932      932              
  Partials       181      181              

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[sandorkertesz]

@HCookie, many thanks for adding these features backed with proper testing! Once the code is updated with the latest develop it can be merged.