collection module not accessible from the top module


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['karate']

In Python, a module (even a submodule) needs to be imported before it
can be used, so you need to do:

   import graph_tool.collection['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['karate']