simplify halo mass sampling

[ntessore]

I am having a hard time following if and how skypy.halos.mass is supposed to be used, given that there is also skypy.halos.colossus_mf.

Ideally, there would be a <model>_mf function for sampling halo redshifts and masses for each model, similar to schechter_lf. This would also be ideal if we decide to move away from COLOSSUS in the future.

Then we could remove the sampling from skypy.halos.mass and have that submodule only contain functions that deal with the mass property of halos, but not the sampling.