support pickling of models and components
support pickling of models and components

Weakrefs in components were causing a problem, but we can easily recreate them
from the top (Model) down upon unpickling.

johnbachman/pysb,johnbachman/pysb,spgarbet/pysb,neurord/pysb,alubbock/pysb-legacy,LoLab-VU/pysb,d-fan/pysb,alubbock/pysb-legacy,alubbock/pysb,d-fan/pysb,pysb/pysb,neurord/pysb,alubbock/pysb-legacy,neurord/pysb,d-fan/pysb,LoLab-VU/pysb,alubbock/pysb-legacy,spgarbet/pysb,jmuhlich/pysb,spgarbet/pysb