Re-enable healpix interface in yt after resolving licensing concerns

[yt-fido]

Originally reported by: Bili Dong (Bitbucket: qobilidop, GitHub: qobilidop)

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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, 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?

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• Bitbucket: https://bitbucket.org/yt_analysis/yt/issue/1048

[yt-fido]

Original comment by Nathan Goldbaum (Bitbucket: ngoldbaum, GitHub: ngoldbaum):

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Reimplimenting the healpix functionality constitutes a new feature and shouldn't block 3.3.

[qobilidop]

I'm starting to work on this. I'm not very familiar with the volume rendering infrastructure. I notice that there're camera.py and old_camera.py. My understanding is that old_camera.py is deprecated in favor of camera.py, and what used to be old_camera.py/HEALpixCamera, should now be reimplemented as lens.py/HEALpixLens, am I correct? @ngoldbaum @MatthewTurk

[ngoldbaum]

Yup, that’s right.