Stricter enforcement of unmet demand and excluded_commodities

[tsmbland]

Currently there are checks in place to make sure that commodity supply is sufficient to meet demand across the system as a whole. If this isn't the case, a (very subtle) warning message is raised, and the program is allowed to continue.

Users can exclude certain commodities from this check by using the excluded_commodities list in the settings file. In many models there will be valid reasons to do this, if there are commodities that are consumed but with no corresponding supply process (e.g. wind, solar).

In most cases, if the warning message is raised, it probably means the user has forgotten to add a commodity to excluded_commotidies. If that's not the case, and the program is still raising the warning message, then this is probably something that should be taken seriously.

My suggestion is to be stricter about enforcing this. Rather than raising a subtle warning message, terminate the program and raise a helpful error message pointing people to the excluded_commodities list. This would help bring attention to any more serious problems, which may otherwise fly under the radar, and it also forces users to have a bit more of an understanding about the structure of their model.

Users will still have the option of allowing some unmet demand with the tolerance_unment_demand parameter.