AccretOnPlanets

We have no test on this routine (it can be easily added to examples/3D_isothermal_disk)

anyway it looks like /physic/planetary_system/planets/BigEart/accretion_time

is active only if accrete_onto_planets is true

while we need to have them independently to have (disk.cpp):

if (physic().planetarySystem.accreteOntoPlanets){
        if (planetPhysic.feelDisk) {
          p.setVelocity(Triplet(PxPlanet/Mplanet,
                                PyPlanet/Mplanet,
                                PzPlanet/Mplanet));
        }
        p.setMass(Mplanet);
      } else {
        system().loosesAccretion(dMplanet);
      }

I will make a branch for it,

I have also a question for @alainm : what is the good way to have the code writing dMplanet and what system().loosesAccretion(dMplanet) does?