grdfilter option to do local LS plane

Added by Paul about 4 years ago

This was an old idea that got reintroduced by Joe in msg772: He wants to fit a LS plane to all points inside a radius R, then remove the plane evaluated at the node, and subtract from data, i.e. a high-pass filter. Should we consider
-Ft<diameter> for trend filter? It would return the node-value of a LS plane. To get what Joe wants just give a negative filter width.