Raise warnings for uncalibrated models

[tsmbland]

Thanks for this. I think zero capacity should only be allowed (for the base year) when demand is also zero for the base year. The idea is that modellers calibrate the base year - i.e. they tell the model what the existing capacity is for any process that gas capacity in the base year. If they enter a capacity value that is not sufficient to meet demand in the peak time slice, then the model should throw a helpful error telling them that they need to calibrate the base year.

Originally posted by @ahawkes in #427