The otree class implements the oct tree data structure, used for
calculating the potential energy of a system of point masses. The
data structure, and its utility in N body simulations, is discussed
by Barnes and Hut 1986 (Nature v324 p446).
The oct tree recursively partitions a 3D cell into 8 subsells. New
points are added to the tree such that each point corresponds to a
leaf, and all points are separate from one another.
July 2010: Written by Chris Beaumont