Set number of qubits for leakage detection

[sjdilkes]

Description

The automatic leakage detection tries to maximise parallelism by using as many spare device qubits as possible for performing leakage detection. Currently when passing leakage_detection=True to process_circuit/s, it automatically assumes that the user wants to make use of all qubits. This PR adds a new kwarg to pytket-quantinuum n_leakage_detection_qubits that allows the user to specify fewer qubits.

Related issues

Please mention any github issues addressed by this PR.

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]

I think we should update the changelog as well.

[sjdilkes]

I think we should update the changelog as well.

Just done!

[yao-cqc]

Thanks!