Added super cool awesome 'subset' method.  You send in a list of pores from the current network and it returns a new network of those pores.
Final touches on 'subnet'
- When the method is called it not only returns subnet to play with, but also puts labels in the main network indicating which pores (and throats) correspond to which pores in the subnet.
- See the doc string on the 'subnet' method for more details, but basically pn.pores(sn.name) gives the list of which pores in the main network were used in the subnet work.
Merge branch 'develop' of https://github.com/PMEAL/OpenPNM into develop

Conflicts:
	OpenPNM/Network/__GenericNetwork__.py