Implement diffusion constraints by technology groups and across regions

[danielhuppmann]

The formulation should be extended to incorporate diffusion constraints that include multiple technologies or work across spatial or temporal scales.

See the following article for a previous implementation in MESSAGE: Benjamin D. Leibowicz, Volker Krey, and Arnulf Grubler. Representing spatial technology diffusion in an energy system optimization model. Technological Forecasting and Social Change 103:350-363, 2015. 10.1016/j.techfore.2015.06.001