Implement a photon-response-non-uniformity (PRNU) model

Photon-response non-uniformity is very simply the fact that every pixel on a detector array has a slight different QE, or photon-to-charge conversion efficiency. The user should be able to define a map of QE for their detector.

• Implement a user-definable QE map. This could be a file upload or a pixel matrix
• #677 (closed) Implement a wavelength-dependancy for the QE map. This could be a function applied to a single uploaded map, (e.g. an AR coating transmission curve) or an interpolation between a few maps at different wavelengths.