Added count to GenericGeometry.  This now means that the number of pores in a geometry can be found with all the same tricks as networks and fluids (Np,Nt,num_pores,num_throats, and now count)
- This was basically cut-and-paste, but it's only a few lines so not worth figuring out how to inherit it
Merge branch 'develop' of https://github.com/PMEAL/OpenPNM into develop