Add optional gradients to linear interpolant

[MichaelClerx]

Easy addition to linear interpolant. Will be discontinuous at nodes. Can just pick positive direction for both (2 point deriv, not averaged from current & last line segment!)

[EricWay1024]

I'm considering doing this before making a NLopt example, so that the example can make use of gradients. Looking into the code for linear_interpolant, I wonder

• if we can calculate using grad_x = (h21 - h11) / self._resolution and grad_x = (h12 - h11) / self._resolution?
• where the gradient method should be placed? Should we create another class with __call__ defined, or make it a method of linear_interpolant?

[EricWay1024]

if we can calculate using grad_x = (h21 - h11) / self._resolution and grad_x = (h12 - h11) / self._resolution?

Seems that this calculation method is fine after testing it with a gradient-based method in NLopt.

[MichaelClerx]

Closed in #68