We need a public available output for further processing of ImageStatisticsContainer:

Two choices:

1. Implement Iterator interface to iterate over all statistics
2. Function to retrieve keys and function to retrieve values for each key:
   • std::vector<std::string> GetKeys() const in a defined order
     • each key is a defined constant (no evil magic string!)
   • boost::variant GetValuesByKeys(std::string) const
   • optionally possibility to put value into std::string
     • but not neccesarily in ImageStatisticsContainer class
     • regard that std::to_string() ignores locale control. use ostringstream instead.

as payload, use std::map<std::string, boost::variant> (multiple types have to be stored as value: double, unsigned int, itk::index...).