Sclopt patch, allows multiple subnetworks and pre-existing constraints

[TomAdamsHET]

Closes # (if applicable).

Changes proposed in this Pull Request

Allows naming of constraints for separate subnetworks, and allows existing constraints to carry through

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[fneum]

It's a nice idea, but I would not handle it in this way by default. The risk is that components added between the first model creation and this function call may be ignored:

n.optimize()
n.add("Generator", "gen", ....)
n.optimize.optimize_security_constrained()

I would also replace:

if "model" in dir(n):

with

get(n, "model")

But I would consider adding this as an optional keyword argument of the function.