Introduce sigma_v + v_off, and remove line FWHM as a user-settable parameter

[jdtsmith]

A real ambiguity regarding the Model -> Features interface is what type of data is returned back into the Features table. The most principled approach is to return only details of the scientific model. The FWHM of a line is not such a detail.

Rather than letting users set that for unresolved lines, and having Model populate it after fit, it would be a cleaner approach if lines (and dust features) allowed an optional sigma_v (in km/s). sigma_v (default units: km/s) represents additional line width above and beyond that of the (Gaussian) instrumental line profile.

There are some details about applying sigma_v, but it's by far then the cleanest approach: all Features tables remain "unsullied" by artifacts of the instrumentation, and can hence be applied to any spectrum(/a) you throw at it (as long as they have valid instrument information)

Then fit, tabulate, plot, etc. would not need a way to "override the instrument". They are always applying instrument details of the plotted/tabulated/fitted spectrum(/a).