Gradient finders should be encapsulated in a polymorphic class

[geronm]

We will soon have several different methods for inverting from GelSight RGB to gradients. These different methods have different initializations and invocations, and so it would be nice to have them encapsulated in classes where the constructors would handle initialization (and paramaterization, if there is any) and the class would have a method convertRGBToGradients for the inversion.

Inversion methods we have so far:

• convolution filter applied to BGSub'd image
• spatially-varying lookup table applied throughout the RGB image
• octree-based XY->RGB inversion
• lookup bin RGB->XY mapping

EDIT: added last two inversion methods