threshold.py: bug fix for overlapping buffers in output

There are sometimes overlapping buffers in the output of the threshold element. This is because the simplify() method is only called on the list of TSSlices added to self.nongap_slices, but there might already be slices in the self.nongap_slices itself, which is not simplified with the newly added TSSlices.

This bug was not identified until now because although the _TSTransSink base class sanity checks continuity of frames here: https://git.ligo.org/greg/sgn-ts/-/blob/main/src/sgnts/base/__init__.py?ref_type=heads#L275, it doesn't sanity check the continuity of buffers inside the frames.