feature_extraction: add fake live data source (white_live), add special case for default gps start time since fake live sources do not seem to be seekable or able to change gps time offsets, add seeking for fake sources. fxtools/utils.py: fix issue with feature saving directory structure when gps times are less than 10 digits

- If EOS is reached before the required number of FFTs are computed to make transfer functions, a transfer function will be written with the available data
- Added option to take a median of transfer functions instead of averaging them with a mean
- Added option to store the first transfer function computed after a gap, so that it can be used after the next gap.

used for the cosmic string search trigger generator
see commit a34aa048

a more streamlined version of the conventional lalapps_StringSearch

- adjust axes limits
- add commented out code for dumping CSV file of data