---------------------
Added
^^^^^
- Add ``add_grouped_edges_from`` that adds edges in groups.
- Add ``add_returns`` as additional input to the model. The parameter is used to
output intermediate values in the returns.
- Add ``tox`` command for different python version test environments: py38,
py39, coverage, and docs. The latter two check test coverage and build
sphinx docs.
Changed
^^^^^^^
- Change node attribute ``rts`` to ``returns``.
- Change ``add_linked_edge`` to ``add_grouped_edge``.
- Change ``add_edge`` and ``add_edges_from`` updates graph edge attributes.
- Move ``mmodel`` build method from ``setuptools`` to ``poetry``.