Blurb::
Stopping critiera based on L2 norm of gradient
Description::
The \c gradient_tolerance control defines the                                             
threshold value on the L2 norm of the objective function gradient
(for unconstrained minimum or no active constraints) or the
Lagrangian gradient (for constrained minimum) that
indicates convergence to a stationary point. 
The \c gradient_tolerance control is currently defined 
only for the OPT++ and ROL libraries.

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