[yt-dev] Issue #1048: reimplement grid_traversal.pyx (yt_analysis/yt)

[Bili Dong]

New issue 1048: reimplement grid_traversal.pyx
https://bitbucket.org/yt_analysis/yt/issues/1048/reimplement-grid_traversalpyx

Bili Dong:

In **grid_traversal.pyx** the following 10 functions will raise a **`NotImplementedError`** no matter what:
* `hp_pix2vec_nest`
* `arr_pix2vec_nest`
* `hp_vec2pix_nest`
* `arr_vec2pix_nest`
* `hp_pix2ang_nest`
* `arr_pix2ang_nest`
* `hp_ang2pix_nest`
* `arr_ang2pix_nest`
* `pixelize_healpix`
* `healpix_aitoff_proj`

Based on the discussion in [Issue #1047](https://bitbucket.org/yt_analysis/yt/issues/1047/radial-column-density), this is because their common dependency `healpix_interface` was removed for licensing concerns. I've only seen those functions being used in the module **/yt/visualization/volume_rendering/camera.py**. So I guess either those functions should be fixed, or **camera.py** should be rewritten.

If my understanding is correct, the module `yt/utilities/lib/healpix_interface.pxd` should also be removed, right?