STREL Create morphological structuring element.
SE = STREL('arbitrary',NHOOD) creates a flat structuring element with
the specified neigbhorhood. NHOOD is a matrix containing 1's and 0's;
the location of the 1's defines the neighborhood for the morphological
operation. The center (or origin) of NHOOD is its center element,
given by FLOOR((SIZE(NHOOD) + 1)/2). You can also omit the 'arbitrary'
string and just use STREL(NHOOD).