TODO: provide a shortcut to directly set SNR (rather than exposure, etc...)

[tlambert03]

Both @rayanirban and @federico-carrara would benefit from a more direct way to specify the SNR of the output image, without having to futz around with things like light power and exposure time.

Not immediately sure where the best place to set this would be. It could be a simulation model parameter that defaults to None, and, when set, overrides some other settings; it could also be something that is only accessible via a parameter to one of the methods like digital_image() (similar to the photons_pp_ps_max that was removed in #74)