maximum kernel width and maximum error in the Smoothing app

The Smoothing application, in gaussian mode does not expose the MaximumKernelWidth (Maximum allowed kernel width for any dimension of the discrete Gaussian approximation) and the MaximumError (The maximum error of the gaussian blurring kernel in each dimensional direction) of the DisceteGaussianImageFilter from ITK.

It would be a nice addition to add more control on gaussian filtering in otb