socketio: do not go back to NEEDINIT

The socketio interface currently goes back to NEEDINIT after returning the forces, but that is unnecessary. Make it go to READY instead. That should allow the driver to continue with fewer steps and is going to be assumed by i-pi.

• Check that this works with i-pi as a driver

Closes: #1915 (closed)

Checklist: x (done) or ~ (irrelevant)

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Changes have been added in changelog.d using scriv.
• Issue is resolved via "closes #XXXX" if applicable.