Fix QasmWriter.add_multi_bit() for non-register-aligned arguments

[cqc-alec]

Description

Don't assume the arguments in add_multibit_op() are register-aligned.

With this change, if the arguments aren't all whole registers we write out the operation as individual bit operations.

Related issues

Fixes #1327 .

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]

Thanks, it seems that this has also fixed #1509 :)

[cqc-alec]

Thanks, it seems that this has also fixed #1509 :)

Cool! Will let you close that one if it is fixed after merging this.