[IMP-dev] Particle removal and deactivation

I can't find the imp-commit email for some reason. Anyway, there was a recent patch (581) changing a model::remove_particle to deactivating the particle. This change is wrong as the particles are currently reference counted in C++ and removing the particle from the model should do the right thing assuming there are no python references hanging around. If it does not, it is a bug in kernel code which should be fixed.

In fact, the "Particle::set_is_active" method should probably go away.