getPartialDerivativeIndex
Get the index of a partial derivative in the array.
If all orders are set to 0, then the 0th order derivative is returned, which is the value of the function.
The indices of derivatives are between 0 and size − 1. Their specific order is fixed for a given compiler, but otherwise not publicly specified. There are however some simple cases which have guaranteed indices:
the index of 0th order derivative is always 0
if there is only 1 freeParameters, then the derivatives are sorted in increasing derivation order (i.e., f at index 0, df/dp at index 1, d2f/dp2 at index 2 … dkf/dpk at index k),
if the order is 1, then the derivatives are sorted in increasing free parameter order (i.e., f at index 0, df/dx1 at index 1, df/dx2 at index 2 … df/dxk at index k),
all other cases are not publicly specified.
This method is the inverse of method getPartialDerivativeOrders.
Return
index of the partial derivative.
Parameters
derivation orders with respect to each parameter.