New feature region grouping v2

[djwels]

Description

Added new set for user defined region groups.

User can assign nodes to a region group and set national level targets (e.g. emissions reduction, RE targets etc.)

Tested at the moment for emissions reduction in region groups. Results validated and map to budgets set for a region group with two nodes and one with one node

Tested on a larger 3 node PHL model and solved within a reasonable time.

Checklist

• Dependencies install correctly in a clean environment and code executes;
• Test coverage extended; created tests fail without the change (if possible);
• All tests passing;
• Commits follow a type convention (e.g. https://gist.github.com/brianclements/841ea7bffdb01346392c#type);
• Extended the README, documentation and/or docstrings, if necessary;