Ensure that input commodities are never produced and output commodities are never consumed

[alexdewar]

Currently we don't check either of these things.

Am I right in thinking that this is something we should check @tsmbland?

[tsmbland]

Yes I would say so!

[tsmbland]

In theory we're changing input/output commodities to "other", but this will still apply in the sense that these commodities can be consumed or produced but not both.

[alexdewar]

Yep. I'll take this one along with #603.

[alexdewar]

I have written some code for this, but it occurred to me afterwards that there could be places in the code where it'll be useful to know whether a commodity is an input or an output, in which case it might make sense to keep the distinction (at least internally, even if we change the file format).

The one that came to mind was calculating prices, though I guess in both cases it would probably just involve some combination of commodity levy + flow cost(s), so maybe it's fine.

Can you think of anywhere else we might need to know if these commodities are consumed/produced @tsmbland?