Get more information about the number of realizations in full enumeration

It is interesting to be able to infer the number of realizations generated by a full enumeration of the GSIM logic tree without actually performing the full enumeration. This info will be needed for https://bugs.launchpad.net/oq-engine/+bug/1327027
The naming is a bit misleading: 'realizations' to me is connected to some random process. Here you are doing a deterministic process. I would use the world: 'branches' simply.
what about renaming the method: 'get_num_branches'?