DMGS equation 3.3, f2 term
gev_f2f(y, v1, d1, v2, fd2, v3, d3)
3d array
a vector of values at which to calculate the density and distribution functions
first parameter
the delta used in the numerical derivatives with respect to the parameter
second parameter
the fractional delta used in the numerical derivatives with respect to the parameter
third parameter
the delta used in the numerical derivatives with respect to the parameter