Since every call of p=IMP.Particle() is immediately followed by m.add_particle(p) (and at this point there is no way to make it so that a particle could be in multiple models) I'd suggest that we add a constructor of Particle that takes a model allowing everyone to save a line here and there.

The usage is so asymetric between particles and restraints and things that I don't think our original rationale for having the two calls be separate holds.

We can leave the old one for backward compatibility (but hide it in the documentation and examples).