Flush convergence callback

[je-cook]

Description

The implemented convergence callback for pyvmcon doesnt flush the print statement therefore in some situations the printing won't appear on stdout until a while later.
This enforces flushing after every update.

I have no idea how you would test this so I haven't.

Checklist

I confirm that I have completed the following checks:

• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[timothy-nunn]

Cheers James, do we have any idea why this was happening? Was it when PROCESS was being run as a subprocess?

[je-cook]

Cheers James, do we have any idea why this was happening? Was it when PROCESS was being run as a subprocess?

I was running in a subprocess yeah. It sometimes happens if stdout is redirected in some way. Seen it happen on our side too previously when we weren't running a subprocess (in another context not with PROCESS).