Implement accessor

[malmans2]

• Closes Introduce xarray accessor #34
• Tests added
• Passes pre-commit run --all-files
• Changes are documented in whats-new.rst
• New functions/methods are listed in api.rst
• Project, label, and assignee tabs are populated

MWE:

import pydomcfg
from pydomcfg.tests.bathymetry import Bathymetry
ds = Bathymetry(1.0e3, 1.2e3, 1, 1).flat(5.0e3)
ds.domcfg.jpk = 31
ds_zco = ds.domcfg.zco(ppdzmin=10, pphmax=5.E3)

[codecov]

Codecov Report

Merging #36 (df29e62) into main (6cf13c5) will increase coverage by 0.80%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #36      +/-   ##
==========================================
+ Coverage   94.05%   94.85%   +0.80%     
==========================================
  Files           4        5       +1     
  Lines         185      214      +29     
==========================================
+ Hits          174      203      +29     
  Misses         11       11              

Flag	Coverage Δ
unittests	94.85% <100.00%> (+0.80%)	⬆️

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Impacted Files	Coverage Δ
pydomcfg/domzgr/zco.py	92.38% <ø> (ø)
pydomcfg/__init__.py	75.00% <100.00%> (+3.57%)	⬆️
pydomcfg/accessor.py	100.00% <100.00%> (ø)

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[malmans2]

@pyNEMO/pydomcfg This should be good to go and ready for review.
It just adds the final layer that we expose to users.