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

[Cameron Hummels]

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