Center of mass versus center of magnification

[jenniferyee]

For binary lenses, we did find the origin as the center of mass. We also allow the user to specify point_source_point_lens as a magnification method. For the case of wide binaries, the difference between the center of mass and the center of magnification may be a problem when switching between the point_source_point_lens and binary lens magnification methods. We should definitely check this and maybe write a unit test for it.

[rpoleski]

Maybe we should add a keyword origin or reference_frame to ModelParameters().

Honestly, when I need only a reparametrization, then I use re-parametrization available in example 16: see changes in this branch. So far, all functions I needed, are available in ex16.