Update ImagingExtractor

[pauladkisson]

This PR is the first of a series that implement breaking changes to fully standardize the roiextractors api and update/streamline the testing suite. Some of the main goals include

• removing channels argument to enforce 1 channel/extractor
• Switching to modularized mixins for testing and supporting pytest fixtures/parameterization
• Standard input validation, numpy indexing for 'get' methods, etc.

This PR focuses on ImagingExtractor, FrameSliceImagingExtractor, and NumpyImagingExtractor

[pauladkisson]

A roadmap for future PRs for this dev effort:

• ImagingExtractor: Update ImagingExtractor #365
• SegmentationExtractor Update SegmentationExtractor #366
• BaseExtractor: Add Base extractor #367
• Testing and tools: Tools #369
• MultiImagingExtractor, VolumetricImagingExtractor: Compound extractors #371
• Each individual extractor will get its own PR implementing standards (ex. CaimanSegmentationExtractor, HDF5ImagingExtractor, etc.)

[h-mayorquin]

Hey, this escaped my radar. Let's have a call once you are back from the retreat to have a brief discussion before I do a full review.

[h-mayorquin]

What do you say of peeling this appart to get smaller improvements through? I think that Ben will be on board with removing writing capabilities.

I would also like to remove get_number_of_channels #391.

What do you say if you take the first (removing writing) and I take the second (removing channel related methods)?

Then I think what's left will be the renaming?

[pauladkisson]

What do you say if you take the first (removing writing) and I take the second (removing channel related methods)?

Sure!

Then I think what's left will be the renaming?

And I also revamped the tests to make them much more modular.