[yt-dev] Issue #1040: Enhancements to ray objects (yt_analysis/yt)

New issue 1040: Enhancements to ray objects https://bitbucket.org/yt_analysis/yt/issue/1040/enhancements-to-ray-objects Cameron Hummels: * Make annotate_ray() optionally overplot an arrowhead on the ray for illustration of directionality * Include `vector` attribute on `OrthoRay` and `Ray` objects * Allow easy creation of `LightRay` objects from `OrthoRay` and `Ray` objects with a method for automatically doing this. * Make annotate_ray callback in new VR interface using `LineSource` Responsible: chummels