d11: Computing each element of the function DD_weight_1
In Opt5PL: Optimal Designs for the 5-Parameter Logistic Model

This function computes each element of the vector function DD_weight_1 which is the first derivative of the Ds-optimality criterion with respect to the model parameters.

1	d11(T, x, xl, inv, inv1, order)

`T`	A numeric vector. Model parameter values
`x`	A numeric. ith 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 for the 5PL model
`inv1`	A numeric matrix. Inverse of the information matrix for the 4PL model
`order`	numeric. The number of model paraemters