Allow generic window control for Rayleigh spectra

Created by: alurban

This PR fixes #1242 (closed) by allowing users to select generic window functions when calculating Rayleigh spectra. As with other spectral density estimation methods, any window format recognized by scipy.signal.get_window is acceptable.

Note, Rayleigh spectra are given a hard 0 seconds of overlap between segments by default. Users will be able to either choose their own overlap, or pass None to get scipy's default based on the window function. This is reflected in various docstrings.