First complete release of the rewritten GCAM Data System. Produces datasets necessary to run GCAM 5.0.

This release is the product of a lot of work by many people–see DESCRIPTION file–but Kalyn Dorheim @kdorheim Abigail Snyder @abigailsnyder Ryna Cui @rynacui and Russell Horowitz @russellhz all need to be particularly thanked.