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 `GetValuesByKeys(std::string) const`
as value type, use `boost::variant` as multiple types have to be stored: double, unsigned intboost::variant GetValuesByKeys(std::string) const` (optionally possibility to parse as string, index...but not neccesarily in `ImageStatisticsContainer `class)
also:
- typedef std::vector < std::pair<std::stringas payload, use `std::map<std::string, boost::variant>` (multiple types have to be stored as value: `double`, `unsigned int`, std::string> > statisticsOrderedVectorType`itk::index`...).
- use map?