Add _debug_fill_halos() for arbitrary halo recalculation

[Sfonxu]

No description provided.

[slayoo]

https://github.com/slayoo/PyMPDATA/blob/7b9b980dfab233e0ff111f66e889273b4edd1999/tests/unit_tests/test_boundary_condition_extrapolated_2d.py#L18-L34

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.85%. Comparing base (9872aa7) to head (e9b6950).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #502      +/-   ##
==========================================
+ Coverage   91.79%   91.85%   +0.06%     
==========================================
  Files          30       30              
  Lines        1474     1485      +11     
==========================================
+ Hits         1353     1364      +11     
  Misses        121      121              

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