Create `agent_cost_limits` table

[tsmbland]

To do after #453

See #442 for details of the new proposed schema for agent parameters

Steps:

• Create AgentCostLimit struct
• Create read_agent_cost_limits and read_agent_cost_limits_from_iter functions to read in the data
• Create cost_limits attribute of Agent (replacing current capex_limit and annual_cost_limit attributes)
  • This could be a vec<AgentCostLimit>, or possibly as a hashmap with commodity id as the key
• ALTERNATIVELY: store this under the AgentCommodity struct that will be created in Create agent_commodities table #453. This should all become clearer after that PR is done

Validation:

• Only permit agent/commodity combinations specified in agent_commodities
• If an agent/commodity combination is specified for one year, it must be specified for all years (at least for now, however see [UMBRELLA] Allow "all" option for year and region #444)
• Presumably capex_limit and annual_cost_limit should be non-negative