ENH: Rename noise keyword argument in redshifts_from_comoving_density function

[rrjbca]

It was noted in the review for our JOSS paper that the keyword argument noise for the function redshifts_from_comoving_density has a misleading name. See the review comment here. They suggested renaming to fixed_N. My own suggestion would be shot_noise (or perhaps poisson). Since this would require a breaking API change, this should only be made as part of a future major release (if at all).