Add PixelResponseNonUniformity (PRNU) detector effect

[ShannonS00]

Add a PixelResponseNonUniformity effect to model the fixed, pixel-to-pixel variation in detector quantum efficiency (PRNU). The effect multiplies the image by a 2D map of per-pixel deviation, drawn from a normal distribution with a small standard deviation.

This effect should be:

• Multiplicative (scaled with signal level)

• Fixed (the same pattern applies to every exposure (it is a property of the detector, not random noise))

• applied before ShotNoise so PRNU modulates the true signal before Poisson sampling

• Different behaviour for the separate detectors (H2RG & GeoSnap) are planned.

• New class PixelResponseNonUniformity planned in scopesim/effects/electronic/

name: prnu
  class: PixelResponseNonUniformity
  kwargs:
    prnu_std:  (exact values TBD)
      H2RG: ~0.5 - 1 % 
      GeoSnap: ~1–3% 
    random_seed: "!SIM.random.seed"

The implementation is oriented on the dark current structure.

A companion FlatFieldIllumination effect (large-scale illumination gradient / vignetting) will be added.

[ShannonS00]

Please let me know if you have any comments!