Checks if the solution has converged to within the specified
            tolerance.

def _check_convergence(self, F):
        """ Checks if the solution has converged to within the specified
            tolerance.
        """
        normF = linalg.norm(F, Inf)

        if normF < self.tolerance:
            converged = True
        else:
            converged = False
            if self.verbose:
                logger.info("Difference: %.3f" % (normF - self.tolerance))

        return converged