Add helper functions for dicts
Add helper functions for dicts

* dict_flatten - flattens nested dictionaries
* flatten_log - flattens nested dictionary and joins tuple key with a
  seperator.

joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue,joaander/hoomd-blue