Make sure injected L3 sources include Poisson noise

[schlafly]

I had forgotten that if non-chromatic sources are used, add_objects_to_image doesn't include Poisson noise and simulates a noise-free image.
https://github.com/spacetelescope/romanisim/blob/main/romanisim/image.py#L215-L221
This leads the L3 source injection approach to not actually inject Poisson noise. We should probably add an option to add_objects_to_image to include Poisson noise.