Ajout tp système planétaire

.gitignore

practical_work/electric_fl/enonce.md

practical_work/planets/Exemple_force.svg

practical_work/planets/Makefile

practical_work/planets/MathJax.js

practical_work/planets/enonce.md

practical_work/planets/perihelie.svg

practical_work/planets/skeleton/Makefile

practical_work/planets/skeleton/gfx/gfx.c

practical_work/planets/skeleton/gfx/gfx.h

practical_work/planets/skeleton/main.c

practical_work/planets/skeleton/planet/planet.c

practical_work/planets/skeleton/planet/planet.h

practical_work/planets/skeleton/vec2/vec2.c

practical_work/planets/skeleton/vec2/vec2.h

practical_work/planets/skeleton/vec2/vec_tests.c