added model methods to get a few useful parameter subsets
added model methods to get a few useful parameter subsets

closes #19

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