SYNOPSIS:
    NLGS_FIELD field axis sigma sigmaw [factor]

DESCRIPTION:
    Do a separated NLG 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,
    sigmaw the deviation of the gaussian in the value domain.
    Factor can be used to stretch the searched window. It defaults
    to 2*sigma.
