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).