Added a network_health method, analogous to data_health.  It checks for pores with no throats, and distinct clusters.  This is meant to analyze integrity after trim operations.
Refinements to network_health; it now returns a named tuple with the details of which pores are isolated and which pores are part of each disconnected cluster
Merge branch 'feature/NewPNM' of https://github.com/PMEAL/OpenPNM into feature/NewPNM