Update to read SI pixelscales directly from pysiaf

[mperrin]

A small improvement in precision: use the measured pixel scale values from on orbit when computing PSFs. This should (slightly) improve the computed PSFs.

• Read pixel scales directly from pysiaf for each instrument and detector. Removes having "default" pixel scales of each SI hardcoded in the Python; instead it reads from the SIAF values during init.
• Update pixel scale when you switch to a different SIAF aperture (in particular, if you switch NIRCam detectors it will change the aperture to the one for that detector, which will update the pixel scale).
• Update unit tests correspondingly to still work with the above.

Note, this still retains the idealized assumptions of (a) square pixels, same scale in X and Y, and (b) all of NRC, FGS, NIS detectors are precisely unrotated with respect to the OTE V frame. The non-ideal departures from that, as recorded in pysiaf, continue to be handled in the rotation step and distortion model used to make the OVERDIST and DET_DIST extensions 3 and 4.

This also retains the ability to override the pixel scale with a custom value. If the user does this, then that custom value will be retained afterwards even if you switch apertures (i.e. setting a custom pixelscale disables the behavior for auto updating the pixelscale).

[mperrin]

@obi-wan76 FYI this is a PR that I submitted in December, a simple bite-size standalone minor improvement.

[mperrin]

Huh, this was working in December. Something seems to have broken in the rebase onto current develop. I'll investigate.

[mperrin]

Updated and improved. This now works as intended to automatically read pixelscales from SIAF, per each aperture, but also to retain the ability for a user to set a custom pixelscale (and have that custom pixelscale not be overwritten by SIAF values).

This is now ready for review @obi-wan76

Example:

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.07 🎉

Comparison is base (26ba1b4) 55.21% compared to head (fb5ca14) 55.28%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #626      +/-   ##
===========================================
+ Coverage    55.21%   55.28%   +0.07%     
===========================================
  Files           14       14              
  Lines         6122     6132      +10     
===========================================
+ Hits          3380     3390      +10     
  Misses        2742     2742              

Impacted Files	Coverage Δ
webbpsf/webbpsf_core.py	73.91% <100.00%> (+0.21%)	⬆️

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[obi-wan76]

Looks good, I was able to run the example code and some other apertures.