Modify computation of the gradient norm

Remove division by sqrt(dim) in the computation of Stats.GradientNorm
because:
* The name of the field is misleading otherwise
* Stats.GradientNorm is reported by Printer which then also becomes
  misleading
* The division is unnecessary; dimension of the problem is readily
  available if someone needs the divided value
* Using such modified gradient norm seems non-standard in gradient
  convergence checks
Add the max-norm of the gradient to Stats
Use the maximum norm in the gradient convergence test

... and report it in Printer.
Remove computation of the 2-norm of the gradient