copied from stampede.
on stampede, dynamic threading seems working.
particle class has mass.
centroid calculation added.
parallel task scheduling seems working.
however, it seems that openmp does not distribute the tasks very well so I am trying to implement it manually.
evaluate_trees function seems working correctly so far.
now I have to implement actual force calculation part.
everything working, but the evaluate tree takes significantly longer than others.
evaluate_tree needs work.
target is not necessary in the finest leaf.
done for tonight.
stampede not working...
before going to school
Merge branch 'threading'

Conflicts:
	prob_5/Makefile
	prob_5/nbody.C
	prob_5/particle.C
	prob_5/particle.h
	prob_5/qtree.h
	prob_5/test.m