fix airy amplitude scaling

[mileslucas]

• fix airy amplitude scaling (fixes Numerical issue with obscured airy disk #14)
• fix moffat FWHM conversion
• add tests

[mileslucas]

@sefffal Turns out I had the math completely wrong- this change will be breaking for both the Moffat and Airydisk functions. If you want to give a pull and check it works like you want you can, otherwise I'll merge and register in a day or two.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.21%. Comparing base (feaa43c) to head (9abf813).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #15   +/-   ##
=======================================
  Coverage   99.21%   99.21%           
=======================================
  Files           6        6           
  Lines         128      128           
=======================================
  Hits          127      127           
  Misses          1        1           

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[sefffal]

Looks good, thanks for the quick fix @mileslucas!
I haven't done a side by side comparison to see what changed, but the output looks good to me and fixes the issue I noted.