remove Measurement.to_graph_ground and unused ground parameter

[dlyongemallo]

Description

Remove the representation of measurement as a node with a ground symbol, as this causes confusion and conflicts with the representation of ground using symbolic boolean values.

This is a breaking change.

Related issue

Follow-up to #402 (comment).

Checklist

• I have added/updated tests (for bugfixes, please include a regression test, for new features, include new tests either to an existing test file or a new file).
• For new features: I have updated the documentation.
• All the functions I added have a docstring parsable by sphinx (for a good example see pyzx.extract.extract_circuit).
• I have added an entry to CHANGELOG.md describing my change.

[Copilot]

Pull request overview

This PR removes the “ground-symbol” graph representation for measurements and standardizes measurement graph conversion to always use the symbolic-boolean (symbolic phase) representation, to avoid confusion/conflicts with ground vertices.

Changes:

• Removed Measurement.to_graph_ground and the ground parameter path from Measurement.to_graph.
• Deleted tests that specifically asserted the removed ground-mode measurement behavior.
• Added a changelog entry documenting the removal.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
pyzx/circuit/gates.py	Deletes Measurement.to_graph_ground and makes Measurement.to_graph always emit the symbolic-boolean representation.
tests/test_qasm.py	Removes tests that depended on the now-removed ground-mode measurement behavior.
CHANGELOG.md	Notes the removal of Measurement.to_graph_ground in the Unreleased section.

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