integrate spectral lines for column densities, abundances, etc.

[jbudynk]

Iris should have a specific tool/action which calculates the area inside an absorption/emission line. A user could currently do this by integrating under the continuum component and spectral line(s) separately, then subtracting the continuum integral from the spectral line integral. However, the user has to do the subtraction by hand. It would be nice to provide a specific functionality so that the user can do this more easily.