improves BNG path detection
improves BNG path detection

* now $BNGHOME is examined as well as the hard-coded path list

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