Given a 2D array as input, return a copy in which there is a band
adjacent to the edge with zero gradient normal to the edge
data (input, 2D array)
The input array, can be any data type
WIDTH (input, integer scalar)
The width of the band in which values are changed. Default is 1.
If WIDTH is less than 1, the result is just a copy of the input.
The function returns an array of the same type and dimensions as
Copy interior values into the band. Note that the band width is
the same on all boundaries: given this, there is no need for
special handling at corners.
Mark Hadfield, 2006-03:
Written to replace mgh_moma_top_smoothedge and similar.