Compound extractors

[pauladkisson]

Awaiting #369 then will change base to 0.6.0dev.

This PR is the fifth 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 updates MultiImagingExtractor and VolumetricImagingExtractor and adds appropriate tests.
Originally, I was intending on updating MultiSegmentationExtractor as well, but none of the current segmentation extractors use this class, so its utility/requirements are very unclear rn.