divergence_theorem.c running,  numerical check
In a 3D volume, F is a vector function, e.g.
F(x,y,z) = sin(x) i + cos(0.5*y) j + (z^2+1)/7 k 
The volume chosen is a cube with dimensions:
xmin=0.000000, xmax=1.000000
ymin=0.000000, ymax=1.500000
zmin=0.000000, zmax=2.000000
volume integral((del dot F)dv)  should equal
the sum of 6 surface integral((F dot normal))ds 

nx=10, dx=0.100000, ny=10, dy=0.150000, nz=10, dz=0.200000
integral grad F(x,y,z) dx dy dz = 2.845860 
surface xy0=-0.214286, xy1=1.071429
surface xz0=-2.000000, xz1=1.463378
surface yz0=0.000000, yz1=2.524413
volume=2.845860, surface=2.844934, err=9.263541e-04

nx=20, dx=0.050000, ny=20, dy=0.075000, nz=20, dz=0.100000
integral grad F(x,y,z) dx dy dz = 2.845165 
surface xy0=-0.214286, xy1=1.071429
surface xz0=-2.000000, xz1=1.463378
surface yz0=0.000000, yz1=2.524413
volume=2.845165, surface=2.844934, err=2.315349e-04

nx=40, dx=0.025000, ny=40, dy=0.037500, nz=40, dz=0.050000
integral grad F(x,y,z) dx dy dz = 2.844991 
surface xy0=-0.214286, xy1=1.071429
surface xz0=-2.000000, xz1=1.463378
surface yz0=0.000000, yz1=2.524413
volume=2.844991, surface=2.844934, err=5.788036e-05


divergence_theorem.c finished