Generalize n(z)/p(z) parameterization

[aimalz]

As qp shows, redshift posteriors and redshift density functions can be parameterized in many possible ways. chippr unfortunately was scoped out before I'd really thought about a catalog that might not use the piecewise constant parameterization of SDSS. KiDS has photo-z PDFs parameterized as Gaussians and wants n(z) with way more parameters than chippr can actually handle. This issue can be closed when:

• chippr can accept Gaussian photo-z PDFs without forcing them onto a grid
• chippr can yield n(z) in a different parameterization than the input photo-z PDFs