@contextmanager<EOL>def session(auth):
sess = Session(auth)<EOL>yield sess<EOL>
Create a session context to avoid explicitly passing authentication to 
every function.

Example:

.. code-block:: python

    import yaxil

    auth = yaxil.XnatAuth(url='...', username='...', password='...')

    with yaxil.session(auth) as sess:
        aid = sess.accession('AB1234C')
        experiment = sess.experiment('AB1234C')
        sess.download('AB1234C', [1,3,14], out_dir='./dicomz')

:param auth: XNAT authentication
:type auth: :mod:`yaxil.XnatAuth`
:returns: YAXIL session object
:rtype: :mod:`yaxil.session.Session`