Add a border attribute to cnecs

[phiedw]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines
• [x] Tests for the changes have been added (for bug fixes / features)
• [x] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem?

No

What kind of change does this PR introduce?

Feature

What is the current behavior?

Cnecs do not have a border attribute

What is the new behavior (if this is a feature change)? They now do (FlowCnecs, VoltageCnecs and AngleCnecs)

Does this PR introduce a breaking change or deprecate an API?

• [ ] Yes
• [x] No

What changes might users need to make in their application due to this PR? (migration steps)

They may now add adapt importers to add the border information if they so wish.

Other information:

This is done for the cse swa process, where the dichotomy algorithm needs to know to which border each cnec belongs to, to know where to do counter trading.

[phiedw]

Documentation needs to be added.

[phiedw]

The csa importer could probably be updated as well to start using this.