ENH: Different ways of differentiating

[Gui-FernandesBR]

We need to improve the Function.differentiate method. As we know, its implementation is not ready for production and should be enhanced

See #122 as an example of where it is mentioned

[giovaniceotto]

Suggestions:

• Complex step differentiation
  • Reference 1
  • Reference 2
• Automatic differentiation
  • Reference 1
  • Reference 2
• Finite differences schemes as a fall back (let the user specify the order)
  • Reference 1