Optional analytical Jacobian.
Given the parameter vector, it returns a function that, for an x value,
yields the partial derivatives of the model with respect to every parameter
(an array of length params.length, in the same order as params).
When provided, it replaces the finite-difference approximation of the
Jacobian, which is both faster (no extra model evaluation per parameter)
and more accurate (so the optimizer usually needs fewer iterations).
Optional analytical Jacobian. Given the parameter vector, it returns a function that, for an x value, yields the partial derivatives of the model with respect to every parameter (an array of length
params.length, in the same order asparams). When provided, it replaces the finite-difference approximation of the Jacobian, which is both faster (no extra model evaluation per parameter) and more accurate (so the optimizer usually needs fewer iterations).