This class stores a list of mass model objects.  Given the eta values defining the geometry of the system it can ray trace observed positions onto each mass plane defined.

This class stores a list of light models and given a ray-traced positions for each light plane will calculated the surface brightness of each light plane and return them as a stack.

This class is ued to forward model a multi-plane lens system.  This class adds new features of the current single-plane case:
- Only use mask boundaries for pixel grid extents (for speed)
- Only ray trace the observed positions once (and feed the results into the extent method if needed)
- add source_grid_scale to scale the resulting extents of a pixel grid by a constant factor
- add conjugate_points as a convenience function to quickly trace a know list of points to each source plane.

To do:
- add doc strings
- make sure all features work as expected

The number of planes axis will always be -2 coming out of vectorize.