Flag out NaN weights in coadd

[keflavich]

I encountered a severe error case in which the weights array became NaN after reprojection. Flagging out locations where weights is NaN solved the issue.

I cannot come up with an MWE; this occurs deep in the guts of a complicated mosaicking attempt, and it happened for only 1 field out of 270. I think it might be the consequence of a numerical error that occurs only under unique conditions, but it also looks logically like this fix should work.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.32%. Comparing base (2ab5757) to head (2f67e7d).
Report is 45 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #474      +/-   ##
==========================================
- Coverage   91.26%   89.32%   -1.95%     
==========================================
  Files          25       25              
  Lines        1065     1077      +12     
==========================================
- Hits          972      962      -10     
- Misses         93      115      +22     

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