Calculate Jacobian of mu and xi w.r.t the vector [z, y]

[chenwilliam77]

Nowhere in the code is it required to calculate the Jacobians of mu and xi w.r.t. z and y separately. We only need the Jacobian of mu w.r.t [z, y]. This approach would also allow caching of variables required to exploit sparsity in Jacobians (via SparseDiffTools.jl).