Added a file for density models
Added a file for density models


Former-commit-id: 64d528220c086c8f3d1e17f8bdf8ef262b868698
Former-commit-id: 4d04d29e9271c42fca75c587e21bdf9957bec06a
PMEAL/OpenPNM,TomTranter/OpenPNM,amdouglas/OpenPNM,stadelmanma/OpenPNM,amdouglas/OpenPNM