Add support for Virgo FFL files

[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.

[basswinkels]

Attached the last 10 lines of raw.ffl and trend.ffl (with .txt added for github). Note that in case of trend, there are frames with mixed length.

The files contain 5 columns, which are: gwf_filename, gps_start, duration, unknown, unknown. The last 2 are usually 0, this might be something related to files containing triggers, I will ask.

trend_sample.ffl.txt
raw_sample.ffl.txt