Refactor/agent types

[cpnota]

This PR makes Agent and ParallelAgent distinct types. This is because Agent.act(state) accepts a State whereas ParallelAgent(state_array) accepts a StateArray, meaning that one cannot be substituted for the other, i.e. the Liskov substitution principle is violated if they are treated as the same type. Similarly, this has the ripple effect of making Preset and ParallelPreset distinct types, as well as PresetBuilder and ParallelPresetBuilder.

There are also a few other minor changes:

1. PresetBuilder objects imported by users are instansiated by default. E.g., what was previously a2c().device('cuda') should now be written a2c.device('cuda').
2. The name attribute is now passed from the Builder to the Preset, which will be used to improve logging.
3. The Preset base classes were extended to store the name, device, and hyperparameter attributes which are shared across all presets, improving code reuse.
4. Addresses Preset import structure #199 to make presets easier to copy/paste into custom presets