Use cdshealpix for margin pixel finding.

[delucchi-cmu]

Migrates get_margin method to use the cdshealpix.external_neighbours method.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Please upload report for BASE (margin@f732d78). Learn more about missing BASE report.

Additional details and impacted files

@@            Coverage Diff            @@
##             margin     #422   +/-   ##
=========================================
  Coverage          ?   92.86%           
=========================================
  Files             ?       46           
  Lines             ?     2031           
  Branches          ?        0           
=========================================
  Hits              ?     1886           
  Misses            ?      145           
  Partials          ?        0           

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