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
Ok, so this is a method only for the Gsim Logic Tree, can you just add a sentence in the doc, that says that the method assumes a symmetric logic tree (in case in the future we will relax this assumption for the Gsim logic tree)