Add common lib for gst folder and extract flag_segment to it

Related to bypass. In that we're duplicating a lot of postcoh's boilerplate code, including everything related to flag_segments (used to track ifo gaps in a buffer).

In preparation for that, I've extracted flag_segments to a common library in the gst/ directory. There's already some overlap between multiratespiir and postcoh for one function. Multiratespiir has another which is a little too large, multipurpose, and tied in with multiratespiir so I've left it alone for now.

Tests

I've built & set runs going here: /fred/oz016/tdavies/projects/testing/run_tdavies__extract_flag_segment_type
Test results: test_zerolags_8_12_extract_flag_segment_type.txt
Note I was comparing with tiebreak_clustering, so there's the same differences with end_time_sngl_ getting initialized to 0 as in the snr_series change.

Even though the changes are minor, it should be beneficial to show there's no change on this MR prior to the larger bypass changes.