Define the convergence threshold. It computes the difference between the "input"
field in the iterative procedure, and the "output" field. If this difference is
less than OCTEps the iteration is stopped. This difference is defined as:
(If there are several control fields, this difference is defined as the sum over
all the individual differences.)
Whenever this condition is satisfied, it means that we have reached a solution point
of the QOCT equations, i.e. a critical point of the QOCT functional (not
necessarily a maximum, and not necessarily the global maximum).