Transfer Function can now be changed with a graph

Transfer Function graph not final size and location and obscures much of the 3D model view now. Still not able to change the colors of the color map gradient. While merging i noticed that the colors don't match from the bounding box and the slice, not sure why. (see image below)

Left-Click to add control point and click and hold to move control point, UI can be fine-tuned later

Added a separate TransferFunctionGraph class for handling the graph and TransferFunction class for the actual TransferFunction which gets sent to transfertexture and then applied to current colormap, unsure of this is best solution, but works well.