ENH: Add "Count" ivar for zombie book-keeping

Added "Count" ivar to use for counting the number of
times a zombie halo is propagated to the next timestep.
ENH: Add zombie cutoff parameter

Added ZombieCutOff threshold parameter and logic. Now, when zombies
are being propagated more than N times, they are designated as
zombies and are not being checked for REBIRTH.
ENH: Add book-keeping for zombie handling

Added functionality for keeping a count of the number
of times a zombie halo is propagated to the next
timestep.
BUGFIX: Fix MergerTree percent threshold

Changed percent threshold to be '>='
ENH: Add "REBIRTH" event
ENH: Detect Rebirth events
BUGFIX: Fix majority rule for similarity

The majority rule used for detecting similarity between two halos
at different timesteps should compute the percentage of the particles
in the current timestep that belong to the halo at the previous timestep.
STYLE: remove debug output
BUGFIX: Reset count of zombie halo
ENH: Rollback I/O code
ENH: Rename DIYHaloItem to HaloInfo
WIP: Redesign