Implement relevant missing features from v2 in v3

[jessicasyu]

As we transition from v2 to v3, we want to make sure relevant features from v2 are captured in v3. In general, agent/environment classes are largely unchanged but need work with the v3 style of input/output/configs. Visualization and simulation classes will require larger changes, but may not all be necessary to port over (see #19 to help scope which features are relevant to keep vs. which features we can drop).