Added a 'find_duplicates' method to GenericNetwork
- It finds duplicated pores (spatially overlapping) or dupicate throats (connected the same two pores)
- It can either (a) return a list of offenders, or (b) remove them from the network
Removed a bug in 'count' that  both pore.all and throat.all to be present
Merge branch 'develop' into feature/better_props