Add constraint to mask out polar horns

[lpsinger]

Fixes #81.

[lpsinger]

@cenko, here is map of the value of the new constraint, at an altitude of 600 km. Does this look about right? I see bands that I recognize as the polar horns, but I see another blob that is probably the SAA.

[codecov]

Codecov Report

Merging #82 (d245402) into main (c139e64) will not change coverage.
The diff coverage is n/a.

❗ Current head d245402 differs from pull request most recent head 3ca746f. Consider uploading reports for the commit 3ca746f to get more accurate results

@@           Coverage Diff           @@
##             main      #82   +/-   ##
=======================================
  Coverage   28.78%   28.78%           
=======================================
  Files          35       35           
  Lines        1779     1779           
=======================================
  Hits          512      512           
  Misses       1267     1267           

Impacted Files	Coverage Δ
dorado/scheduling/mission.py	93.75% <ø> (ø)

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[cenko]

Looks good.