Fix Xpress IIS mapping for masked constraints

[FabianHofmann]

Closes #603

Changes proposed in this Pull Request

• fix Xpress IIS label mapping to use self.matrices.clabels so masked constraints are mapped back to the correct linopy constraint labels instead of dense solver row positions
• add and strengthen the masked-constraint infeasibility regression test to verify matching constraint names and coordinates in the reported IIS
• clean up related setter and variable docstrings and add a release note for the Xpress IIS fix

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.