Treat RICH < 0 like RICH = 0 in shear (PP) mixing

Add if statement when computing Pacanowski-Philander mixing coefficients to set
Mdiff and Tdiff as though RICH = 0 when RICH < 0. I also removed the "no_diff"
optional argument to cvmix_coeffs_shear; I believe can't find any notes or
correspondence supporting the existence of this flag, and if we really need the
capability to update nu without changing kappa I think there several other
approaches to consider that are far less klunky (e.g. making Tdiff an optional
argument).