The TREE method

1. Subdivision of the considered volume in virtual cells. Daughter cells have exactly half dimension of their mother cell.
2. In real calculation it is necessary that the subdivision stops. Principle:
   1. Empty cells are discarded
   2. For cells containing exactly one particle, the mass and position of the particle is stored (in `theelement')
   3. For cells with more than one particles, subdivision is performed in daughter ccells. In addition, center of mass, mass and quadrupole moment (if needed, also higher moments) are stored (in `centroid')
3. This TREE is newly calculated in every time step (where the whole considered volume is taken as starting mother cell)

       vector acceleration (particle, ensemble) {
         if singleton(ensemble)
           acc = newton_acc (particle, theelement(ensemble))
         else if
           ( diameter(ensemble) / distance(particle, centroid(ensemble))
             < theta )
           acc = newton_acc (particle, centroid(ensemble))
         else foreach(e, ensemble,
           acc = sum_vector (acc, acceleration (particle, e)) );
         return acc;
       }