Re-introduce knitro context closure and export solver quantities

[finozzifa]

Closes # (if applicable).

Changes proposed in this Pull Request

The Knitro Python API stores in the knitro context object all quantities related to a problem optimization. The API is built in such a way that the quantities can be extracted with dedicated methods only from the knitro context.

In order to make the linopy interface more flexible and avoid to extract in linopy all the relevant quantities, I exposed in PR #600 the knitro context and removed the lines that freed it once the optimization was completed (in fact when a knitro context is freed, all the information it contains is lost).

This however proved to be problematic in terms of memory usage.

Therefore, in this PR, I am not exposing anymore the knitro context, I free the knitro context and compute the necessary quantities from within linopy and export them as part of the Result object.

FYI @danielelerede-oet

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).
• 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.

[FabianHofmann]

@finozzifa could you add a release note?

[finozzifa]

@finozzifa could you add a release note?

Done!