Robust chain in apply mode goes in an infinite loop

[Ulrich A. Mbou Sob]

This is a tiny issue I need to fix when I find some time. The line below will make the solver go into an infinite while loop if we are running in apply-only mode. The work around for now is never to use the robust solver if are only applying gains.
https://github.com/ratt-ru/CubiCal/blob/master/cubical/machines/jones_chain_robust_machine.py#L101