Turns on/off the use of the update norm (eq. 4.42) as a way of determining convergence. The default is off.
Turning this on is useful if the behavior of the residual indicates
numerical truncation error (i.e. if the residual reduces rapidly,
but then bottoms out at a level insufficient to satisfy the residual
convergence tests). This behavior can show up with very small time
steps (TR/BDF2 and the concentration form of the generalized
trapezoid family subtract nearly equal vectors that are proportional
to 
) and nearly equilibrium reactive-diffusive systems.