Prescribe config dimension in CSV inputs

[chrisroadmap]

In IAMC-style inputs, particularly forcing ones in which the scale factor has no impact (#126 #166), an optional index could be added to the forcing (and possibly concentration and emissions) CSV files that describe the config number to which a time series relates. This blurs the lines a little bit between a scenario and a config. But allowing different time series for different configs of the same scenario would allow exploration of emissions uncertainties.

Additionally in the parameters file the index should be labelled "config".