add attribute to Spectrum class to hold un-assigned columns of HDU1 BINTABLE.

[kelle]

Many JWST data products provide multiple columns in the BINTABLE of HDU1. Currently, the readers figure out the relevant columns for wavelength, flux, flux_uncertainty, (and mask?) and don't do anything with the unassigned columns. I propose we add an optional attribute to the Spectrum class to hold these unassigned columns (or the entire original BINTABLE) as an Astropy Table.

The case study would be any x1d file from JWST. Here's the description https://jwst-pipeline.readthedocs.io/en/latest/jwst/data_products/science_products.html#extracted-1-d-spectroscopic-data-x1d-and-x1dints but I'll try to find a good file to use a test case.