Update Noise Aware Placement for Circuits with only single qubit gates

[sjdilkes]

#922 brings up that qubits without 2-qubit gates aren't placed by NoiseAwarePlacement. This is fine for GraphPlacement as these qubits will be dynamically assigned during routing, with their allocation unrelated to performance as it assumes "no noise", but for NoiseAwarePlacement they could still be assigned to Node in the Architecture that have smaller error rates.

However, the expectation is that typical circuits won't have qubits without any 2-qubit gates, i.e. this is a special case fix. My proposed solution is:
• If the Circuit has qubits with 2-qubit gates and qubits without 2-qubit gates, don't assign the qubits without 2-qubit gates to any Node, as these Node may be better used as dynamically assigned later.
• If the Circuit only has qubits without 2-qubit gates, then assign them to the Node with best reported error-rates.

This changes two previous tests that were seeing how NoiseAwarePlacement worked on circuits with no 2-qubit gates. In particular, these tests assumed they would be assigned to the unplaced register, while now they are assigned to any Architecture Node. I have considered this a fine change & minor change in behaviour as this not an impactful change for any realistic circuits.

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.

[cqc-alec]

Just a typo in the changelog.