Second-order bivariate differentiation calculation.
SecondOrderMixedDiffVal(value.c, value.x.p, value.x.m, value.y.p,
value.y.m, value.xy.p, value.xy.m, epsilon)function value at (x,y).
function value at (x+epsilonx,y).
function value at (x-epsilon,y).
function value at (x,y+epsilon).
function value at (x,y-epsilon).
function value at (x+epsilon,y+epsilon).
function value at (x-epsilon,y-epsilon).
offset value for both x and y.
(value.p -2*value.c + value.m) / (2*epsilon).