Improve Agent/Agent Methods Coverage

[olivia-banks]

Relevant Codecov:

• https://app.codecov.io/gh/UofUEpiBio/epiworldR/blob/main/R%2Fagents-methods.R
• https://app.codecov.io/gh/UofUEpiBio/epiworldR/blob/main/R%2Fagents.R

To Bind:

• get_state
• get_agents
• agents_from_edgelist
• get_network
• get_agents_states
• add_virus_agent
• add_tool_agent
• has_virus
• has_tool
• change_state

Concordant with #77.