specutils: various features

-Continuum fitting, subtracting, and normalizing. I use specialized slit trace fitting and running median filters for my work but it seems to be far from a solved problem and can be very data dependent. It would be nice if there were an easy way to do it.

-Cosmic ray removal! I try to use the python version of LA-cosmic most of the time but it would be nice to integrate it or similar algorithms into astropy.

-Interpolating to position-velocity space (complete with error propagation and flux conservation)

-More general propagation of the errors (variance). It is my understanding that this is now a feature included in astropy (or at least under development), which I don’t think was the case when I stated writing plotspec.

-Extracting line fluxes, especially for 2D spectra. Plotspec can do weighted optimal extractions or use region files from DS9 drawn onto PV diagrams. It can also extract fluxes from 1D spectra given some velocity range for the line widths.

-First order sky emission removal. Especially for OH lines in the NIR. I take two off frames and subtract one from another, and then do a chi-sq minimization which seems to correct for linear variation across a set of spectra okay. Would be nice to generalize, at least for IR astronomers.

-Constructing data cubes and all the really obscure WCS astrometry stuff that comes with that. This might be beyond the scope of astropy.

requests come from Kyle Kaplan