Implement data structures for entire dataset slicing

[blairlyons]

Use Case

As a computational user, I want to be able to slice AgentData by time and agent in a computationally efficient way.

Acceptance Criteria

Implement design from #35

Potential Reviewers

(while Blair is away)
@meganrm @toloudis @jessicasyu

[ascibisz]

Summary of research in deciding what to do is here: https://docs.google.com/document/d/1FHU7QRxWijQyJEIrJgOstkTWsoAtq7Caph1uDHPUQ9Q/edit#heading=h.wufvdleokyrg