added beginning of BH. must revamp figs+

slides/cours_21.md

slides/figs/arbre1.png

slides/figs/arbre2.png

slides/figs/arbre3_1.png

slides/figs/arbre3_2.png

slides/figs/arbre3_3.png

slides/figs/corps1.png

slides/figs/corps2.png

slides/figs/corps3_1.png

slides/figs/corps3_2.png

slides/figs/corps3_3.png

slides/figs/nbody_bare.png

slides/figs/nbody_group.png

slides/figs/nbody_n2.png

slides/figs/nbody_qt.png

slides/figs/nbody_qt_withtree.png