Decision objects don't belong in production

[taylorhansen]

Currently, Networks create Decision objects whenever it makes a decision. This is only necessary during training, so the code that currently pertains to that should be removed.

Instead, in train.ts, provide a means to subclass into a TrainingNetwork that's able to properly create those Decision objects with the help of having full control over the simulation in the play() function.

The additional control can also be used to hook into events and replace the purpose of BattleAgent#acceptChoice(), #onFaint(), and #onTurn(), as well as moving the RewardTracker class into scripts/ and not as a field of Network. As such, the BattleState#agent field will be useless so that can be removed.