Several changes to fix line subtraction when lines turn on and off:

 - Stop subtracting signal when lines are off
 - Don't use GstController for line subtraction updates due to detection of secular changes in TFs
 - Simpler measurement of secular change in line subtraction TFs
 - lal_insertgap: new property open intervals to allow bad-data-intervals to be open or closed intervals
 - Distinguish between zeros and missing data in line witness channels
 - lal_property: fix for occasional segfaults
 - lal_smoothkappas: Use property reset_times instead of reset.
 - lal_stdev: Use property reset_times instead of reset.
 - lal_asd: allow for negative fft overlap.
 - gstlal_compute_strain: Option to use a line subtraction gate value, as opposed to a threshold, so that values larger than those that indicate lock (e. g., corresponding to calibration measurement times), can induce gating.