fix mask_ocean for 2D grids

[mathause]

• Closes #xxx
• Tests added
• Passes isort . && black . && flake8
• Fully documented, including CHANGELOG.rst

We are working with the coords so we need to check that the coords are in the DataArray and not the dims.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (91b184b) 87.16% compared to head (7bfa6e7) 87.16%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #314   +/-   ##
=======================================
  Coverage   87.16%   87.16%           
=======================================
  Files          38       38           
  Lines        1621     1621           
=======================================
  Hits         1413     1413           
  Misses        208      208           

Flag	Coverage Δ
unittests	87.16% <100.00%> (ø)

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Files	Coverage Δ
mesmer/core/mask.py	100.00% <100.00%> (ø)

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