Add timeout parameter to GreedyPauliSimp

[sjdilkes]

Description

Use std::async to add a timeout to greedy_pauli_optimisation.

A segmentation fault was occurring when modifying the python expression held in ClassicalExpBox to relabel the expression bits to match a relabelling.

To work around this, I've added a new argument to Circuit::flatten_registers, Circuit::rename_units and FlattenRelabelRegistersPass to optionally relabel expressions in ClassicalExpBox. This is fine for the greedy_pauli_optimisation case as the synthesis method does not effect the held expressions and we relabel back to the original assignment after re-synthesising the circuit.

Checklist

• I have performed a self-review of my code.
• I have commented hard-to-understand parts of my code.
• I have made corresponding changes to the public API documentation.
• I have added tests that prove my fix is effective or that my feature works.
• I have updated the changelog with any user-facing changes.

[yao-cqc]

Can you add a test for the timeout?

[sjdilkes]

Can you add a test for the timeout?

Do the test changes in predicates_test.py and test_greedypauli not cover it, where I add a 0 timeout and confirm that the false branch is returned? Happy to add more

[yao-cqc]

Ahh I see. Looks great, thanks!