Add support for Virgo FFL files

Created by: basswinkels

At Virgo, the way for the end user to find off-line data is by pointing your Matlab/Python/dataDisplay to a "FFL file" (raw.ffl, trend.ffl, ...). This is a pretty basic text format, where every line lists a file name, gps_start and duration. Reading data via an FFL files is built natively into the FrameL library. As far as I know this doesn't work with gwpy yet. It would be good to implement this, to make gwpy work at the Virgo site.

I will later attach some sample ffl file and possibly some small gwf files.