COMP: Make parameter const char* in Printf methods
ENH: Add mean-center to halo data-structures
ENH: Print mean xyz for each tree node
ENH: Mark halos that are zombies
Create README.md
COMP: Fix minor compilation error on hopper

Fix compilation error when compiling code on hopper with GNU
compilers. The error was due to not being able to resolve off_t
type. This was due to missing the <sys/types.h> include.
Merge branch 'master', remote-tracking branch 'github/fix-missing-include'
Merge branch 'master', remote-tracking branch 'github/master'
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
COMP: Hopper cross-compilation updates

Changed hopper platform CMake to use the default GNU compiler and MPI
library.