[Yt-dev] Particle IO Disabled for now

[Matthew Turk]

Hi all,

I have disabled fast ParticleIO for now.  There's a bug in this code,
which I am struggling to figure out the right way to fix.  This only
applies to Enzo data, and the bug only showed up when you had a
derived field depending on only one field, and only if it's more than
a simple conversion factor.  The only field I found that this applied
to was ParticleAge.

During the reading, we pass in conversion factors for fields, so that
when returned they will be correctly scaled.  However, I was
incorrectly setting dependency fields as the main field; this means
that, for instance, the ParticleAge conversion was being applied to
the creation_time field, and then creation_time was set as
ParticleAge.  This is inside yt/data_objects/particle_io.py.

The problem here is getting all three things playing nicely:

 * Particle IO
 * Conversions
 * Derived fields based on particles

I have to sit back and think about ParticleIO.  Really I have always
kind of hated my particle IO implementation, but I think the solution
is to simplify it and make it behave more straightforwardly.  I'll
notify the list when I've fixed the bug and re-enabled ParticleIO.

Best,

Matt