This function computes each process to WV (haar) in a given model.
derivative_first_matrix(theta, desc, objdesc, tau)A vec containing the list of estimated parameters.
A vector<string> containing a list of descriptors.
A field<vec> containing a list of object descriptors.
A vec containing the scales e.g. \(2^{\tau}\)
A matrix with the process derivatives going down the column
Function returns the matrix effectively known as "D"