Write the latest data file in .zarr format

[simlmx]

• Save a latest.zarr file alongside the latest.netcdf file, the former being faster to read for some applications, but keeping the later for backward compatibility.
• Remove some of the unused functions, and simplify the "save" function.
• Accelerate the unit test for the save function.

[codecov-commenter]

Codecov Report

Merging #62 (5a3bbff) into main (ef36aaa) will increase coverage by 0.16%. The diff coverage is 96.55%.

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@@            Coverage Diff             @@
##             main      #62      +/-   ##
==========================================
+ Coverage   93.52%   93.69%   +0.16%     
==========================================
  Files           6        6              
  Lines         340      333       -7     
==========================================
- Hits          318      312       -6     
+ Misses         22       21       -1     

Impacted Files	Coverage Δ
metofficedatahub/multiple_files.py	93.23% <96.55%> (+0.37%)	:arrow_up:

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