Refactor data transformers to use class objects instead of generator methods

[cboulay]

Generator methods are problematic for a few reasons. For example, they can't be pickled which means they can't be moved across processes using ezmsg's built-in multiprocessing. Additionally, they can't be initialized from scratch with a pre-existing state, which precludes them from use in processors in cloud computing paradigms:

• bytewax
• flink
• Apache Beam

We have decided that we are going to replace the generator methods with picklable objects and with the possibility of initializing them with a pre-existing state. The exact design is yet to be determined so we will go through a design phase which will be informed by some attempts to implement the initial design.

• [ ] Collect and document requirements
• [ ] Refactor several processors in ezmsg.sigproc
• [ ] Iterate and agree on final design
• [ ] Refactor remaining processors in ezmsg.sigproc
• [ ] Refactor processors in ezmsg.core and ezmsg.util
• [ ] Refactor other namespace modules' processors

[griffinmilsap]

@pperanich would you have time in the next week or two to figure out a Protocol proposal here?