PSF interpolation order

[oczoske]

This PR reverts the interpolation method used for the PSF back from RegularGridInterpolator to RectBivariateSpline. The appeal of RegularGridInterpolator was the availability of the pchip method, which however turns out to be not usable for 2D interpolation (excessive memory requirement). RectBivariateSpline is faster.

From a user perspective, the PSF interpolation methods opens up the interpolation order as a user-settable parameter, !SIM.psf.interp_order. For consistency, we also define !SIM.psf.interp_psf (formerly !OBS.interp_psf) to control whether the PSF is scaled to each wavelength slice of a spectral cube or just once to the central wavelength.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.17%. Comparing base (ac9c592) to head (4e5b2f2).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
scopesim/effects/psfs/discrete.py	66.66%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #950      +/-   ##
==========================================
- Coverage   75.18%   75.17%   -0.01%     
==========================================
  Files          70       70              
  Lines        9025     9027       +2     
==========================================
+ Hits         6785     6786       +1     
- Misses       2240     2241       +1     

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