Mmodel

--------------------
Added
^^^^^
- Add ``doc`` attribute for ``MGraph``.
- Add ``draw_graph()`` method to ``MGraph`` and model classes.
- Add ``__repr__`` for ``MGraph`` and model classes.

Changed
^^^^^^^
- Remove ``name`` input for ``Model`` and ``loop_parameter``.
- Generate model names and looped subgraph names automatically.
- Remove ``title`` input for ``draw_graph``.
- Change model attribute ``graph`` to ``G``, to avoid confusion on the graph's
inherent attribute ``graph``.
- Separate ``draw_plain_graph()`` and ``draw_graph()``, the former shows
a simplified version of the graph, and the latter shows all graph details.
- Graph title outputs detailed descriptions of the model instance and
graph instance.
- Node attribute "return_params" to "returns".
- Edge attribute "interm_params" to "parameters".

--------------------
Added
^^^^^
- Add class ``MGraph`` for constructing default graphs.
- Add class ``PlainModel`` for constructing callable from graphs.
- Add class ``Model`` for constructing callable from graphs with
memory management.
- Add class ``H5Model`` for constructing callable from graphs with
h5 data storage.
- Add function wrapper ``basic_loop`` that creates a basic loop for models.
- Add function ``draw_graph`` for drawing DAG graphs.