Added basic gating as TimeSeries method

Created by: tjma12

This PR adds gating as a TimeSeries class method. The method whitens the time series, marks all points above a threshold, clusters them within a time window, and applies inverse Planck windows to zero out the data. The unwhitened, gated time series is returned.

An example can be found here (behind LIGO.org auth): https://ldas-jobs.ligo.caltech.edu/~thomas.massinger/notebook/demonstrate-gwpy-gating.html

Whitening arguments can be passed to the underlying whitening used to locate gating points.