Update volume rendering code to work with more modern yt installations

[jzuhone]

The API for yt's volume rendering has changed completely. This PR updates the volume rendering code for spectral cubes to reflect this.

I haven't tested this yet, so I'm marking it as WIP until I can. In the meantime, I wanted to put it up for @trhunter to play with.

Closes Issue #571.

[coveralls]

Coverage decreased (-3.7%) to 84.309% when pulling 72dd5ac on jzuhone:fix_yt_camera into 31b0b51 on radio-astro-tools:master.

[jzuhone]

Ok, it runs, but the images are blank. This will probably take a little while to dig out of.

[jzuhone]

@keflavich do you have a favorite VR example that we could try out?

[keflavich]

Not really... the GRS l=49 data were pretty good though:

https://spectral-cube.readthedocs.io/en/latest/yt_example.html#sketchfab-isosurface-contours

[jzuhone]

Is this the L1448_13CO.fits file?

[keflavich]

No but that one is totally fine if it looks OK

[jzuhone]

So this is working now. But I'd like to add some tests and change the docs before we merge it.

[jzuhone]

Hi @keflavich this is ready to go except I cannot get the test data I generated to compare to (as .npz files) to copy correctly when installed. I've tried several of the normal tricks but nothing seems to work. Any idea what's going on here?

[keflavich]

in tests/setup_package.py, you need to add *npz

[astrofrog]

@jzuhone - would you be able to rebase this? If you don't have time, I'm happy to take over the PR to clean it up.

[jzuhone]

@astrofrog I can rebase it. I had trouble getting the tests to pass before, but I'll see what happens now.

[astrofrog]

@jzuhone - thanks! When you rebase can you also set the minimum yt version in setup.cfg? If there are any issues with the tests I'm happy to investigate.

[keflavich]

The failure last time was related: https://travis-ci.org/github/radio-astro-tools/spectral-cube/jobs/640107285#L635

We'll need another rebase, and maybe another small correction, to get this fixed.