Include postprocessing of EoS parameters

It'd be good to have PESummary being able to do the postprocessing of EoS parameters from 4PolyEOS, 4SpectralDecomp and BinaryLove. This includes converting the model parameters (for example the SDGammaX parms in 4SpectralDecomp) into Lambda values, as well as NS_radii and other derived params.

ping @leslie.wade as well if you happen to have a student with some time over 🙂