Change the Fmb kernels :

× remove the potentials or forces only
× remove the abstract fmb kernel
× put a attribute to know if we use the mutual P2P or not (a template or inherite model may be faster)

git-svn-id: svn+ssh://scm.gforge.inria.fr/svn/scalfmm/scalfmm/trunk@49 2616d619-271b-44dc-8df4-d4a8f33a7222