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 52.38095% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.60%. Comparing base (06a2d0d) to head (b190dfd).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
scopesim/effects/psfs/discrete.py	52.38%	10 Missing ⚠️

Additional details and impacted files

@@ Coverage Diff @@## main #950 +/- ##
==========================================
+ Coverage 75.18% 75.60% +0.42% 
==========================================
Files 70 70 Lines 9025 8981 -44 ==========================================
+ Hits 6785 6790 +5 + Misses 2240 2191 -49 

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[teutoburg]

A few comments about x and y, but I assume you made sure those are correct, so the comments are more for me to understand.

The indexing issue needs more work. The current implementation seems to produce a 90 degree rotation of the PSF.

[oczoske]

I've checked the coordinate order and made some adjustments. It should be consistent now. Have a look at the attached notebook, too.
Interpolation_coord_order.ipynb

[teutoburg]

I've checked the coordinate order and made some adjustments. It should be consistent now. Have a look at the attached notebook, too. Interpolation_coord_order.ipynb

The notebook fails with No such file or directory: '/home/oczoske/ELT_Development/irdb/METIS/PSF_LM_9mag_06seeing.fits'...

[oczoske]

The notebook is not meant for publication, so just adapt the path. Sorry for the inconvenience.