Add description for PRNU into METIS instrument packages

[ShannonS00]

Complementary to the new PixelResponseNonUniformity effect, a description is added in the METIS detector .YAML files:
Preliminary values are added for the deviation.

Example:
'- name: prnu
description: Pixel-to-pixel response non-uniformity — fixed multiplicative gain map
class: PixelResponseNonUniformity
include: True
kwargs:
prnu_std: 0.5
prnu_seed: 42'