Report absorption features on output of 'save fine continuum' command

Anna Ogorzalek posted a query on the Cloudy group about identifying absorption features in the output of the 'save xspec mtable' command.

A partial answer is to identify features due to the lines alone.

This branch modifies the output of 'save fine continuum' to report all absorption features centered at a given fine continuum energy sorted from most-to-least opaque. Each feature is identified by its spectral label (including wavelength). Its mean optical depth is also reported.