Convert SkyModel to "full" spectral type only when needed

[piyanatk]

Currently in #174 , ModelData converts SkyModel to "full" spectral type at initialization.

hera_sim/hera_sim/visibilities/simulators.py

Lines 78 to 79 in 153eef4

	self.sky_model = sky_model
	self.sky_model.at_frequencies(self.freqs * units.Hz)

If the sky_model attribute is reassigned post-initialization, this line will not be called again, causing an error in the VisCPU call if the re-assinged SkyModel has any other spectral type that is not "full".

hera_sim/hera_sim/visibilities/vis_cpu.py

Line 376 in 153eef4

I_sky=data_model.sky_model.stokes[0, i].to("Jy").value,

Presumably, pyuvsim supports any SkyModel spectral types, so this should not be an issue in the UVSim wrapper class. I have no idea about healvis.

The best thing to do might be to move the spectral type conversion to VisibilitySimulator instances, requiring the developers/users to make sure that the simulators that they want to wrap can handle whatever spectral in the given SkyModel.