DD1: Computing each element of the function c_weight_2
In Opt5PL: Optimal Designs for the 5-Parameter Logistic Model

This function computes each element of the matrix function c_weight_2 which is the second derivative of the c-optimality criterion with respect to the model parameters.

1	DD1(T, x1, x2, xl, inv, p, order)

`T`	A numeric vector. Model parameter values
`x1`	A numeric. ith dose level for a given design
`x2`	A numeric. jth dose level for a given design
`xl`	A numeric. last dose level for a given design
`inv`	A numeric matrix. Inverse of the information matrix
`p`	A numeric. Define EDp
`order`	numeric. The number of model paraemters