Append mode in mod_agg_iohist doesn't work

[rezass]

Hi all,

Currently, darshan-util/pydarshan/darshan/experimental/aggregators/mod_agg_iohist.py:61 checks whether 'agg_iohist' exists in self.data, but this key is never added to data.

The check should be if 'agg_iohist' not in self.summary.