[yt-dev] Unit refactor PR

[Matthew Turk]

Hi all,

Nathan and I sprinted on a few remaining things for the unit refactor,
to the point that I would like to see it get tried out and tested in
production.  Casey and Nathan have done some really cool stuff with
the units work, and I think it will be awesome.

I've issued a PR against the tip of the yt-3.0 branch.  Nathan and I
will likely be updating this throughout the day.

https://bitbucket.org/yt_analysis/yt/pull-request/662/unit-refactor

What works:

 * All of the non-VR *unit* tests
 * Plotting of things
 * Units on arrays!

Each frontend needs to be manually converted, as fields are now
completely redone -- and much nicer.  We have redone:

 * Enzo
 * FLASH
 * RAMSES
 * Boxlib (mostly)
 * Stream

However, before we got too far down the road, I wanted to verify that
things are heading in a direction the developers are happy with.
Specifically, things like the way fields are created (check out
yt/fields/*.py and the frontends/*/fields.py for converted frontends)
and the new UI for dealing with things.

This *needs* to land in some form or another before the 3.0 release,
and I'd like to start getting testing done now so that we can land the
other big item, the rebranding.

Unit refactor can't be accepted until:

 * We get some people to use it, say what's wrong.
 * The other frontends get converted.
 * The VR interface is fixed.
 * Plots have a unit-ful interface, which is mostly done.

So, I'd like to ask that y'all try it out by pulling the bookmark
unitrefactor from my fork ( MatthewTurk/yt ).  On Thursday during the
dev hangout at 2PM EST, this will be the biggest part of the agenda --
actually trying it out.

-Matt