[yt-dev] New default particle union?

[Nathan Goldbaum]

Hi all,

I'd like to propose adding a new particle union that should be defined for
all datasets that include particles. This came up in the context of the
demeshening work (see
https://bitbucket.org/yt_analysis/ytep/pull-requests/67 for more details).

Right now many of the derived quantities make a distinction between
calculating results using just the gas or just the particles or both. Up
until now they have calculated the results for particles using particle
fields from the 'all' particle union. This makes perfect sense for AMR data
but doesn't really make sense for SPH data, since it will double-count SPH
particles. In fact, I think this is an issue even without the demeshening,
but the demeshening makes it more starkly apparent.

I'd like to propose defining a new "n-body" particle union (suggestions for
alternate names are very welcome) that will be defined for all yt datasets.
This union will be identical to the 'all' particle union for AMR data and
N-body particle data, but for SPH data will only include the particle types
that aren't SPH particles (if any). That means the "n-body" particle type
represents infinitesimal particles but not particles that have finite
extents (e.g. an SPH particle's smoothing region).

I think this new particle type would probably be generically useful beyond
just the derived quantities, maybe even more useful than "all". I also kind
of prefer the name "n-body" to "all" since it more prominently indicates
that it's associated with particle data.

Please let me know if you have thoughts or suggestions about this proposal.
I'm happy to draft a YTEP or update YTEP-0031 with more details if people
want to see that.

-Nathan
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