Proposal: Add analyzer.frame_slice

[chrishalcrow]

Following on from #4133, here is a draft proposal for implementing a frame_slice method for the sorting analyzer, to see how tricky it is.

API is:

sliced_analyzer = analyzer.frame_slice(start_frame=1000, end_frame=2000)

which matches the frame_slice methods for sorting and recording objects.

There's a "hard" mode (which just recomputes all extensions) and a "soft" mode (which attempts to estimate the extensions). Almost all the complexity occurs at the level of the extension. For each extension, we'd need to implement a _frame_slice_extension_data method.

I've only implemented _frame_slice_extension_data for random_spikes, and you can only return an in memory analyzer for the moment.

Would like some feedback on the basic structure before doing anything more.