This function is primarily build to make the function components
neater.
doubleintegral(lower_inner, upper_inner, lower_outer, upper_outer, cdf,
...)
The vector for the lower bound for the inner integral
The vector for the upper bound for the inner integral
The vector for the lower bound for the outer integral
The vector for the upper bound for the outer integral
the cumulative density function (from the class "function")
Further arguments passed to the cdf.
a vector with the value of the double integral for each observation (with an observed target variable)