or zncen(ptcen, ireg)
returns zone centered version of the 2-D point centered array PTCEN.
The result is (imax-1)-by-(jmax-1) if PTCEN is imax-by-jmax.
If the region number array IREG is specified, zones with region
number 0 are not included in the point centering operation.
Note that IREG should have dimensions imax-by-jmax, like
the input PTCEN array; the first row and column of IREG are ignored.
Without IREG, zncen(ptcen) is equivalent to ptcen(zcen,zcen).
interpreted function, defined at i0/std.i line 2879