Deal with astropy.cosmology potentially returning Quantity values
Use RectBivariateSpline to speed up flux density evaluation.

* In sed_model, rather than storing an array of interp1d instances,
  use RectBivariateSpline with order 1 to do the 2D interpolation.
  Should be faster.
* Also modify get_fluxes to handle multiple z, u, log10lir values,
  change gencat to use that interface