Disallow having different process flow directions in different years

[alexdewar]

While we check that the primary output for a process is indeed an output for all years for which there are entries in process_flows.csv, other flows are allowed to change from being an input to an output in different years. Unless I'm missing something, I don't think there's a sensible reason why users would need to be able to do this, so I think we should disallow this.

By disallowing it:

• We can catch unintentional mistakes in input files (e.g. missing a minus sign)
• It makes it easier to reason about commodity dependencies across years (I can't see an immediate use for this, but it may help when we're trying to do things like Allow dependency loops between commodities #794)

[alexdewar]

Does this sound right to you @tsmbland? We could also do #929 at the same time.

I'm wondering if it might be worth doing this before some of the other things I was thinking of #880.

[tsmbland]

Yes I think so. I can't think of any good reason users would want to do this

[alexdewar]

It's worth bearing in mind #934 when implementing this. In the longer term, we want users to be able to omit flows information (like availabilities etc.) for non-milestone years as long as there are no assets in assets.csv commissioned on that year. Data should be complete for all milestone years though.