Implement diffusion constraints by technology groups and across regions

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](https://doi.org/10.1016/j.techfore.2015.06.001)

Start by reading the issue and the cited Leibowicz, Krey, and Grubler article, which describes a previous MESSAGE implementation of spatial technology diffusion. Then locate the model formulation entry points and determine how technology groups and spatial or temporal scales are represented; done means the requested diffusion constraints are defined and covered by appropriate model validation.

Written by the indexing model from the issue text.