calculate SI WFE header keyword over just the JWST aperture, not the full array

[mperrin]

Minor fix to header keywords, discovered while cross-checking output PSFs with the lookup table grid.

The header keyword "SI_WFE" should give the RMS WFE evaluated just over the JWST aperture, not the full SI WFE aperture (which may be a circular aperture circumscribing the JWST pupil, or may be a full array with no mask; either way, that's not what we should use)

This has no effect on the output PSFs at all. It just makes one header keyword a bit more accurate.

[codecov]

Codecov Report

Merging #427 (7a4f5db) into develop (7755bc1) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #427      +/-   ##
===========================================
+ Coverage    48.46%   48.50%   +0.04%     
===========================================
  Files           14       14              
  Lines         5877     5878       +1     
===========================================
+ Hits          2848     2851       +3     
+ Misses        3029     3027       -2     

Impacted Files	Coverage Δ
webbpsf/webbpsf_core.py	81.00% <100.00%> (+0.20%)	⬆️

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