int2d.c find volume under sin(x+y)*cos(x*y)  
        for 0 < x < 1   0 < y < 1            
        using two dimensional Gauss Legendre 
n=2, volume= 0.720520927310365, exact= 0.721858383188700, err=-0.00133746 
n=3, volume= 0.721866777098973, exact= 0.721858383188700, err=8.39391e-06 
n=4, volume= 0.721858353212025, exact= 0.721858383188700, err=-2.99767e-08 
n=5, volume= 0.721858383257868, exact= 0.721858383188700, err=6.91682e-11 
n=6, volume= 0.721858383188597, exact= 0.721858383188700, err=-1.02918e-13 
n=7, volume= 0.721858383188713, exact= 0.721858383188700, err=1.26565e-14