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
I would rename in 'get_num_paths'

and in the doc: 'Return total number of paths in the tree.'