Should z_s become a param?

[ConnorStoneAstro]

Now that its possible to link params, we could have it so that z_s for each lens is a param like z_l. Then for a class like SinglePlane we could enforce lens.z_s = self.z_s for all the lenses so that they have the same source redshift (same for z_l really).

[ConnorStoneAstro]

It would also mean that z_s could become more out of the way for most lenses since it could be set at initialization and then ignored afterwards. Right now a bunch of functions need z_s as input even though they don't really need it.