Added the ability to set residual pores and throats.  Residual throats are included in the cluster search since they make invasion easier, otherwise residual pores and throats are just considered as prefilled and cause the capillary pressure curve to start at a non-zero y-value.  Also, shortened the plot method a lot (but still does the same thing).
Added the ability to set residual pores and throats.  Residual throats are included in the cluster search since they make invasion easier, otherwise residual pores and throats are just considered as prefilled and cause the capillary pressure curve to start at a non-zero y-value.  Also, shortened the plot method a lot (but still does the same thing).

PMEAL/OpenPNM,TomTranter/OpenPNM,stadelmanma/OpenPNM