Allow for missing Data Manager 0

[keflavich]

Fix for Issue #29.

Open to suggestions on better ways to interact with iterators here.

[codecov-commenter]

Codecov Report

Merging #30 (89d4a73) into main (987cca5) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #30      +/-   ##
==========================================
+ Coverage   55.39%   55.43%   +0.04%     
==========================================
  Files          16       16              
  Lines        2094     2096       +2     
==========================================
+ Hits         1160     1162       +2     
  Misses        934      934              

Impacted Files	Coverage Δ
...asa_formats_io/casa_low_level_io/casa_functions.py	100.00% <100.00%> (ø)

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[astrofrog]

I'm not sure how the casa functions behave when there are more than one data manager but this seems sensible in principle.

[keflavich]

OK, if you think it's reasonable, let's merge it?