Spectrum1D() should store the best-fit continuum

I believe the Spectrum1D() object needs an optional parameter that stores the best-fit continuum. In the JDAT notebooks, the continuum, once fit, is stored in a separate variable with (coincidentally) the same x-axis as the spectral_axis of the Spectrum1D() from which it was fit. The user needs to keep track of which continuum goes with which spectrum. As a user, I would much prefer to store that best-fit continuum in the Spectrum1D object. Much cleaner, and less prone to error.

Start by reading the Spectrum1D implementation and the JDAT notebooks referenced in the issue to understand how fitted continua are currently represented and associated with spectra. Define the optional continuum storage behavior on Spectrum1D, including how it relates to spectral_axis, and confirm the design with maintainers before implementing it.

Written by the indexing model from the issue text.