Add support for self-gating

This MR adds support for self-gating in pygwb using the module gwpy.timeseries.gate(). This module finds times to gate by whitening the data and using scipy.signal.find_peaks() to find time periods above a configured threshold. These times are then gated (pre-whitening) using an inverse Planck window. The times that were gated and the gate padding are then stored as parameters of the pygwb.detector.Interferometer class. All of the relevant gating parameters have also been added as configurable options in pygwb_pipe.

More details on the relevant gwpy module can be found here: https://gwpy.github.io/docs/latest/api/gwpy.timeseries.TimeSeries/?highlight=timeseries#gwpy.timeseries.TimeSeries.gate