Add Base extractor

[pauladkisson]

Awaiting #366 then will change base to 0.6.0dev.

This PR is the third 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 adds a BaseExtractor class and corresponding BaseExtractorMixin testing mixin to contain commonalities between ImagingExtractor and SegmenationExtractor (mostly methods concerned with _times).