assign_duals() not assigning duals for custom constraints which are not tied to a component type

[ktehranchi]

Describe the feature you'd like to see

I have implemented custom constraints in my model like regional CO2/Capacity/SAFE- and would like to access the duals of these constraints. However- as currently implemented assign_duals() will only assign duals where the component name from (c, attr = name.split("-", 1) ) is a network component or is "GlobalConstraint".

I believe this is already an issue with other pypsa-eur/usa opts like SAFE and CCL where the constraint names are not tied to components.

A temporary solution for myself is to name these constraints as "GlobalConstraint-SAFE_XXX_Region". However, it could be useful to have a dedicated location for these regional/custom constraint duals + a place to store the constraints themselves. I am happy to implement a solution if you all have any preferred location/method for storing these variables.

Or- perhaps just adding in the documentation a note like "if you want to access your duals then set the constraint name to "GlobalConstraint-...."