Added bispectrum and fix burst computation

• #267 Thanks to @tsbinns added feature of bispectrum https://github.com/braindatalab/PyBispectra
• #266 Fixed bug in nm_bursts, s.t. s["bandpass_filter_settings"]["segment_lengths_ms"] restricts threshold computation
• add np.float64 cast s.t. mne notch_filter does not fail with int types