Check internal consistency

[lewisjared]

Describe the solution you'd like
From comment in #22

It would be great to add some way of easily telling if the data is internally consistent or not (e.g. does the same of Emissions|CO2|X add up to Emissions|CO2 and are the regional sums equal to world). I don't know what the best approach is, whether it's a new class (in which case you'd check data was internally consistent before adding it) or adding an attribute e.g. internally_consistent to ScmRun (in which case you could have a method which could check for the internal consistency). I think I'd go for the first because more often than not you know whether you want the data to be internally consistent or not before you start and with the second you run the risk that you start with consistent data, then add new data, then forget to check the internal consistency again and things blow up.