We need a public available output for further processing of `ImageStatisticsContainer`:
Two choices:
1. Implement `Iterator` interface to iterate over all statistics
2. `std::vector<std::string> GetKeys() const` and `boost::variant GetValuesByKeys(std::string) const` (optionally possibility to parse as string, but not neccesarily in `ImageStatisticsContainer `class)
as payload, use `std::map<std::string, boost::variant>` (multiple types have to be stored as value: `double`, `unsigned int`, `itk::index`...).