Changes made from the original serial version of Barnes-Hut code
(obtained from ftp://hubble.ifa.hawaii.edu/pub/barnes/treecode/code_c.tar.Z)

- Modify makefile
  - copied over example makefile
  - added -DDOUBLEPREC to CFLAGS
  - rewrote % directive into a code directive.
  - updated clean target, added distclean target

- added #include <cilk.h>,<cilk-lib.h>, and <cilk-compat.h> to defs.h
- moved code.c to code.cilk
- changed NULL to '\0', as original code didn't typecheck correctly.

Program now compiles, links, and runs.  Not parallel yet.

- made main() and stepsystem() cilk procedures, spawned/sync'd stepsystem
  from main
- added DandC_hackgrav() routine to do hackgrav() calculations in a
  divide-and-conquer fashion.
- moved grav.c to grav.cilk, and made some static variables private.
- moved some variable declarations from header files (code.h and defs.h)
  to code.cilk and declared each shared or private, as needed.
- rearranged statistics gathering.

Program now does the force calculations in parallel.  Now, other
phases of the code are parallelized.

- made particle pushes go in parallel (code.c).
- added lock to cell structure in defs.h.
- moved load.c and io.c to load.cilk and io.cilk.
- made maketree run in parallel.  Redesigned newtree() and makecell()
  to allow parallel allocation of tree cells.
- made loadbody parallel (this is the tough part - uses locks).
- made hackcofm, threadtree, and hackquad parallel.
