Problem using WindTurbineFunctionList (wind_turbines/wind_turbine_functions.py) with autograd for gradient calculation.

The WindTurbineFunctionList __call__ method seems to be incompatible with autograd when passing a list of turbine types. It performs an assignment operation with an autograd arraybox which I think is not allowed - resulting in an error.

A simple case to reproduce this issue can be found below: wt_function_issue_with_autograd.py

This does not happen if I define type = 0, but that wouldn't allow us to run a case with multiple turbine types.