Avoid copying the data into memory
Make messages debug instead of info
Create new MonochromaticFluxes class and adapt fitter to use this

We now always compute sizes on-the-fly: this is because otherwise it’s not straightforward to compute and store sizes for the monochromatic flues, and there is also the danger that the sizes could end up being out of sync with the fluxes, so easier - and fast enough - to compute on-the-fly.
Add support for new model cube in plotting
Removed stellar flux and polarization from SED class.

The stellar flux can be stored in an SED instance of its own, and the polarization can use a PolarizationSpectrum in future.