DESCRIPTION:

Do a separated differentiated linear gaussian filter on
a given axis in a field.
Field may be any dimension, as long as axis fits in.
Sigma is the deviation of the gaussian in the spatial domain.
Amplify is a factor that can be used to scale the result to
a suitable range (default 1.0). Offset can be used to center
the result around a given value, which is useful, if unsigned
data is to be processed.