Add option to pass in a relative scaling instead of peak_flux for klip_fm.inject_psf

*Is your feature request related to a problem? Please describe. Injecting with a peak flux assumes you understand the normalization of the model PSF. Sometimes we haven't understood yet, but know how much fainter than the model PSF we want to inject (a relative scaling).

Describe the solution you'd like Add option to inject psf using a relative scaling factor rather than specifying the peak flux of the injected source.

Additional context For beyond v2.0