PhenoBase and RoiSet upgrade

Modified RoiSet and PhenoBase API:

• added test coverage of PhenoBase client
• simplified handling pipelines that return RoiSets
• removed need to work around accessor and model validation when running pipelines outside of call_pipeline wrapper functions
• PhenoBase now maintains a registry of RoiSets indexed by coordinate dicts
• PhenoBase calls a single add_roiset method when populating both interactively and with batch read method
• a PhenoBase can be empty, in which case it defers setting coordinate keys and patch series until a first RoiSet is added
• RoiSet serializes and deserializes its source image accessor (by reference to pre-existing file, if relevant)
• RoiSet maintains label keys of its classification model(s); PhenoBase makes no assumptions about what these are named
• PhenoBase can output patches by classification class
• refactored how split, sampled PhenoBase subsets are implemented; removed filtering method except in specific case of filtering by classification