From #112. restorePreviousNucleonNumber() and removeNucleonImmediatelyFromAtom() in DecayScreenView both only have model components so they can be moved to DecayModel.
removeNucleonImmediatelyFromAtom uses animateAndRemoveParticle, so let's keep these in the view. I'll make them prototype functions though and see if we like that.
From #112.
restorePreviousNucleonNumber()
andremoveNucleonImmediatelyFromAtom()
in DecayScreenView both only have model components so they can be moved to DecayModel.