Support for cross-spectra

[lamorton]

It would be useful to have cross-spectra (power,phase, coherence) between two signals.

[anchal-physics]

I see that mt_cross_power_spectra is present but there is not simple cross power spectra between two signals like scipy.signal.csd. It would be helpful to do some quick analysis on signals, particularly short time intervals where we do not have the luxury of segmenting or tapering.