Handling convergence issues more gracefully

[PolymerGuy]

Until now, lack of convergence triggers a reference update and a retry. If this does not help, the solver aborts.

We should allow for lack of convergence as this might be a temporary problem and convergence can be obtained a few frames later.

Suggestion:

• Add a "no_convergence_action" setting which can be set to "update", "ignore" or "break".

[PolymerGuy]

This commit implements this enhancement e776697