Improve support for unitless variables in IIASA Projects

[fbenke-pik]

In IIASA Projects like ScenarioMIP there are unitless variables, for example Terrestrial Biodiversity|Shannon Crop Diversity Index. Internal checks like checkVarNames require a unit for variable and piam_variable.

The current solution (#405) is resolving the conflict by adding project-specific exceptions to checkVarNames, but this might not be elegant once more projects require such exceptions.

Alternative solutions could be

• convince IIASA to not accept empty units and introduce unitless for this purpose (Laurin will ask)
• internally use a reserved word (e.g. unitless in our mappings) to satisfy our internal checks, but adjust generateIIASASubmission to not write a unit into the submission to also satisfy IIASA's unit checks
• just stick with the current solution.

[fbenke-pik]

@orichters Let us know if you have encountered this problem before and if not, which solution you'd prefer.