Create an Agent Class

[ThomasThelen]

After the 03/14/21 discussion we want to add some sort of 'Agent' class. It'll probably be more defined over time but off the top of my head it should probably (up for discussion),

have a field for at least,

1. An identifier
2. A list of other agents that it may contain
3. Agent data (maybe we'll make this hold an R6 class in the future)

have methods for

1. adding other agents to its list of agents.
2. Adding a list of agents to the agents
3. propagating through time (it should be an empty function)

There should also be some unit tests testing the constructor and and the relevant functions.