[ENH] - Add signal yielder functionality

[TomDonoghue]

This PR add some helper / utility functions for iterating across arrays. With long time series, in some cases it can be useful to iterate across segments. It adds a function to step across time (for a 1D or 2D array) and a separate function for stepping across 'channels' (across rows within a 2D array).

[ryanhammonds]

Looks good! The I see the usefulness of step_over_time. I'm unsure the use case for:

for sig in step_over_signals(signals):
    pass

This seems more verbose than calling:

for sig in signals:
    pass

Maybe the generator is useful for other cases:

gen = (sig for sig in signals)
next(gen)

gen = step_over_signals(signals)
next(gen)

Anyways this looks good to me!

[TomDonoghue]

Thanks for checking @ryanhammonds!

I agree the time stepper is more useful, I added the channel stepper somewhat for completeness. The use case I was thinking of like the last version you noted - creating a generator at some point, and calling next as needed (I've been doing things like this in the use case that I've been using these in, which is making some gifs).