Add a finite-difference upward derivative

[leouieda]

The upward_derivative function gains a method argument that defaults to finite-diff. The way this works is by upward and downward continuing the grid by half of the average grid spacing and then calculating a central difference approximation for the derivative. An advantage of this method is a lower sensitivity to noise than the FFT-based one. Even though the continuation is FFT-based, it's not very sensitive to high-frequency noise.