generate GPI PSF cube

I wrote a comment in the generate_psf_cube function of the GPIData class saying that same_wv_only = False might not be correct. So it should be checked.

The difference is that same_wv_only combines the sat spots from slices with identical wavelength while same_wv_only = False would combine them all by rescaling them properly. That the rescaling part that might not be correct.

Apparently generate_psf_cube is also not compatible with skipslices... To be corrected.