Extract single TOA as table

[dlakaplan]

Extracting a single TOA as a TOA object does not work. But is there a reason we shouldn't enable a length=1 TOAs object? This allows that, and updates the docs slightly.

Addresses #1352

If this is a bad idea happy to shelve.

[dlakaplan]

Currently includes a warning stating what it's doing. That could be removed it it's too annoying.

[codecov]

Codecov Report

Merging #1381 (481997d) into master (0d6aeaf) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1381      +/-   ##
==========================================
+ Coverage   62.25%   62.26%   +0.01%     
==========================================
  Files          89       89              
  Lines       20267    20270       +3     
  Branches     3653     3653              
==========================================
+ Hits        12617    12622       +5     
+ Misses       6860     6858       -2     
  Partials      790      790              

Impacted Files	Coverage Δ
src/pint/toa.py	83.48% <100.00%> (+0.25%)	⬆️

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[dlakaplan]

Changed warning -> info

[dlakaplan]

I think this is ready to merge as soon as the CI passes.

[aarchiba]

Do we have any functions that expect single TOAs that should now accept single-TOA tables?

[dlakaplan]

I'm not sure. Possibly some of the TOAs construction could do that, but I think you can already construct TOAs from other TOAs.