Create high-level selector API

[micahjsmith]

The tuner provides a high-level propose/add API, in which it maintains an internal state of parameters that have been tried and the associated score, and updates its state in response to these user commands. At a low-level, it uses fit/predict to update its own meta-model.

The selector, in contrast, only provides a low-level API, in which it does not maintain any internal state and requires the user to pass the history of all selections to select.

We should create a high-level selector API, such as "select/add" or "select/record" or "select/report" in which the selector maintains the record of past trials. Then, the current select method can be replaced by the high-level select method, with compute_rewards and bandit already serving as a low-level API.