Calculates the entries of the \(D\) matrix in the definition of the local linear kernel
dij(b,x,y, K)scalar value, the result of \(d_{jk}\).
A vector of design points where the kernel will be evaluated.
A vector of sample data points.
The bandwidth to use (a scalar).
The kernel function to use.
Implements the caclulation of all \(d \times d\) entries of matrix \(D\), which is part of the definition of the local linear kernel. The actual calculation is performed by $$ d_{jk} = \sum_{i=1}^n \int_0^T K_b(x-X_i(s))\{x-X_{ij}(s)\}\{x-X_{ik}(s)\} Z_i(s)ds, $$