Added interpolation to raytrace method to improve smoothness.  Fixes #28
source functions and raytracing now incorporates data cubes.  Fixes #29
build_kappa_map function now allows user to specify sigma,centroid,and axis ratio of an SIElens.
raytrace function now has header
all three lenses now have different parameters through customizing build kappa map function
second lens example now uses RGB (multiwavelength image)