Add ability to plot the full predicted spectrum given a set of model parameters

[ogtelford]

It would be very useful to be able to generate the full model spectrum across all wavelengths (not just in specific filters), e.g., for comparing a best-fit model to available spectral data for the sources of interest. Ideally the wavelength and spectrum could also be saved to a file. This could potentially be added to plot_indiv_fit.py.

[karllark]

See [closed] issue #676 for possible useful code.