have sps and sedmodel objects list their available parameters

[bd-j]

To aid in clarity, the various object classes might store lists of the available fittable parameters. I.e. CSPSpecBasis might have an attribute available_parameters that is a list that includes all the fsps.StellarPopulation parameters as well as things like mass, lumdist, wavecal_coeffs, while the SedModel class would have poly_coeffs and spec_norm.

Transformed parameters would be a bit trickier to incorporate.

[bd-j]

some work on this done in c34b65a8dff