[Feature]: Nested transmission constraints

[gschivley]

Feature description

In addition to transmission constraints between model regions, allow uses to define constraints on total transmission in a single direction between groups of model regions.

Motivation

As regions become smaller, the sum of maximum flow on individual lines is likely to become larger than the maximum flow on all lines between larger regions. An example would be transfer limits between counties in two adjacent states compared to the total transfer limit between the two states.

Implementation strategy

This will require a couple new input files. One to define aggregated regions or combinations of lines. It might be possible to include this combined constraint in the current Network.csv, but the combined line constraints don't have investment costs so it probably makes more sense to have a separate file. This file should accept whatever data a user provides -- if there are multiple nested levels, let that fall out of the data rather than having explicit inputs for different levels of line aggregation.

Additional context

ReEDS has transmission limits at the county, balancing area, and transmission group levels.