Change the allocation of the particles and the way there are stored in the tree.

Now the leafs contains a list of real particles (and not pointers).
It make the P2P running faster (excepted with openmp, I'm going to fix it now).

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/scalfmm/scalfmm/trunk@97 2616d619-271b-44dc-8df4-d4a8f33a7222