returns the zonal volumes of the 2-D cylindrical mesh (R, Z).
If R and Z are imax-by-jmax, the result is (imax-1)-by-(jmax-1).
The volume is positive when, say, Z increases with i and R increases
with j. For example, volume([[0,0],[1,1]],[[0,1],[0,1]]) is +pi.
interpreted function, defined at i0/std.i line 2712