Implement agent cost limits

Start with agent_cost_limits.csv and trace where its capex_limit and annual_cost_limit fields enter the model, then inspect the appraisal optimisation and the flexible outputs discussion in issue #360. Before coding, resolve whether limits apply per commodity or across an agent and how milestone-year gaps and exhausted limits behave; done means agreed constraints are enforced, covered by tests, and the unused-input documentation or warning is addressed.

We have agent_cost_limits.csv with capex_limit and annual_cost_limit fields, but we don't use these anywhere in the model.

I have a vague idea of what these might entail:

• capex_limit: total amount an agent can spend per milestone year (or per year, in the case where there's a gap between milestone years?) on capital costs
• annual_cost_limit: total amount an agent can spend per milestone year on operating costs

Some questions @ahawkes:

• presumably we implement these are constraints in the appraisal optimisation?
• what happens when these limits are reached? No more investment allowed for that agent? In which case, if there's still unserved demand for that agent then it will fail. Perhaps this makes more sense for "compete" commodities, where other agents would potentially be able to make up the rest of the demand
• are these limits for each commodity, or total across all commodities that the agent serves? If the latter, then the order we solve the commodities is important, as commodities solved first could potentially fill up the agent's quota. Perhaps we'd have to solve all the commodities that the agent serves simultaneously, similar to what I've proposed for flexible outputs (#360). That would require all the commodities that the agent serves to be the same rank in the commodities graph - could be a problem, although for most common scenarios I think it wouldn't be

In any case, until we implement this I suggest we temporarily turn off the input file, or at least add a warning to the documentation that this file isn't being used.