Add apd.AnalysisData.__repr__

I really miss this and was expecting something more useful than

>>> datasets
<apd.analysis_data.AnalysisData at 0x7ff9ad0e9370>

I would find e.g.

>>> datatsets
<AnalysisData: WG=SL, analysis=RDs, n_samples=10>

far more useful to the user.

I'm here playing with your example datasets = apd.AnalysisData("SL", "RDs") in the README.

Also the present summary method is a bit dry the way it displays, meaning not very user friendly:

>>> datasets.summary()
defaultdict(set,
            {'config': {'mc'},
             'polarity': {'magdown', 'magup'},
             'eventtype': {'13266069'},
             'datatype': {'2011', '2012', '2016', '2017', '2018'},
             'version': {'v0r0p1735460', 'v0r0p1761304'},
             'name': {'mc_13266069_2011_magdown',
              'mc_13266069_2011_magup',
              'mc_13266069_2012_magdown',
              'mc_13266069_2012_magup',
              'mc_13266069_2016_magdown',
              'mc_13266069_2016_magup',
              'mc_13266069_2017_magdown',
              'mc_13266069_2017_magup',
              'mc_13266069_2018_magdown',
              'mc_13266069_2018_magup'}})