Fixed bug in the 'subnet' method...moved the definition of 'pore.all' and 'throat.all' higher up so that they were available when the other data were set (__setitem__ uses these to compare array sizes against)
Experimenting with subset_fluid method...this seems necessary so that simulations can be run on the subnets, but there may be a better way.
Updated OP documentation, and changed it to use find_clusters from the GenericNetwork, rather than building adj_mats locally.