rl_common is a package of common RL files needed by both the agent and the environment. Mainly the interfaces defining their class methods, and some other utility methods.
core.hh defines the interface used by the agents, environments, models, and planners for them to interact in a standard way. Here are links to each of the important interfaces you may want to implement:
Agent is the interface for RL agents.
Environment is the interface for RL environments.
Planner is the interface for MDP planners.
MDPModel is the interface for MDP models.