[yt-dev] halo merger tree in yt-3.0

[Britton Smith]

Hi all,

I want to address the state of halo merger trees in yt-3.0.  As it stands,
one cannot use yt-3.0 to create a merger tree.  The two existing methods
have not been ported over from yt-2.x and now rely on functionality that
has seen significant changes.  For example, the old HaloProfiler has been
removed, and the outputs of all yt halo finders have been redesigned to
work with the new halo_analysis framework discussed in YTEP-0012.  Both of
these changes break one or both of the current merger trees.  Thus, it will
take significant work to make them functional within yt-3.0.  In addition,
the aim has been to design a new merger tree that would work with the
halo_analysis framework (discussed as HaloCatalogTimeSeries in the YTEP).

In summary, unless there is someone willing to champion them, I believe the
merger trees in yt-2.x have come to the end of their roads.  I could be
convinced otherwise, but given the fact that they are broken at this time,
we need to make a decision by the release of yt-3.0.  I see a few options:

1. Remove the existing merger trees from the yt-3.0 codebase now, but leave
the page in the documentation containing only a note that this
functionality does not yet exist in 3.0, but is still available in 2.x.  I
think there are a few people (including me) who have building a new
3.0-compliant merger tree on their radar so I do not envision us going
without for too long.

2. Keep the existing code and throw NotImplementedYet exceptions when it is
imported.  Additionally, remove all imports to non-existing machinery like
the HaloProfiler.  Add a note to the documentation stating that this no
longer works but is waiting for help to be ported.

3. Fix one or both of them now to at least be functional (if not optimally)
by the yt-3.0 release.  I personally cannot do this.  If you vote for this
option, you should be willing to commit to do this.

I am mostly interested in hearing from people who have a stake in the halo
merger trees, but all input is welcome.

Britton
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