Rewrote Region Node Particle. Started tree constructors

Node and Particle are now simpler and both are pseudoparticles.
Node can have a child particle or child nodes. Position and mass
of a node is defined recursively and represents center of mass.
Region has some helper functions

TODO:
  insertInto node constructor returns an updated node. I don't
  know how the new node with be inserted into the parent.
  figure out the rest of the simulation