Fixes off-by-a-half in FOVs; needs cleaning

[oczoske]

This PR

• replaces scipy.signal.zoom with interpolation using RegularGridInterpolator and WCS in PSF rescaling
• makes sure that the rescaled PSF kernel has even pixel numbers on either side. This is necessary for scipy.signal.convolve with mode='same' keeps the (convolved) source centred in the output.

The second point needs to be refined: Currently it works for FOVs with even pixel numbers, but the code should be prepared to handle odd pixel numbers as well.

Closes #879

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.70%. Comparing base (67be391) to head (09e235d).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
scopesim/effects/psfs/discrete.py	82.35%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #880      +/-   ##
==========================================
- Coverage   74.83%   74.70%   -0.13%     
==========================================
  Files          69       69              
  Lines        8868     8886      +18     
==========================================
+ Hits         6636     6638       +2     
- Misses       2232     2248      +16     

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