I am not quite sure if it is a bug or a design decision (although it seems
quite inconvenient, so I guess it may be a bug), but `collection` module is
not accessible from the top module object.
More specifically, this code does not run:
import graph_tool as gt
gt.collection.data['karate']
As is explained in the documentation, there is a convenience module
called 'all' which imports all submodules, so you do not need to do this
individually for each one:
import graph_tool.all as gt
gt.collection.data['karate']