Move EditAction Behavior to Indexes

[bonham79]

With https://github.com/CUNY-CL/yoyodyne/pull/247, most of the work in instantiating edit actions for the transducer is coming from the Index. It would make more sense to just make this inherited behavior from the Index class and create a new 'ActionIndex` class that takes over all behavior related to edit action creation. This would allow the target_vocabulary related behavior for the transducer to be managed at index creation, instead of adding hacks in the train and model code.

I'm also gonna sneak in potential changes to the expert class that trains the SED when creating the dataloader, allowing us to only iterate through the train set once. With enough code moved around, the expert class can be changed to a mixin (which I'll handle in a separate PR.)