Support waveform/stream stacking

[seisman]

Waveform stacking is frequently used in seismic data processing. Popular stacking methods are:

• linear stack
• Nth root stack
• phase weighted stack

I will try to implement it if it's possible to be included into obspy, maybe obspy.Stream or obspy.signal.

[calum-chamberlain]

Linear and phase-weighted stacks at implemented in eqcorrscan, would be happy to contribute them here if wanted?

[ThomasLecocq]

@calum-chamberlain I'm happy to contribute here too with the/comparing with the pws of msnoise.

[krischer]

This (almost done ;-) ) branch (#1002) also contains these stacks - so work should probably invested into finishing that branch and then generalizing the functionality.

[ThomasLecocq]

👍 for generalizing the stacks outside the array stuff... so investing on #1002 to do that is the way to go !

[trichter]

Implemented in #2440.