Renamed Storage<Type>::element_guard into *::element_reference
* the scope guard is not the element_guard but both the BufferGuard and the File::mirrored_buffer classes
* preparation for introduction of Storage<Type>::container_reference class
Implemented Storage<Type>::container_reference template class
* It is the goal of this class to offer element_reference like functionality for full BinaryMapping::Container instances
* As instantiation of BinaryMapping::Container template specializations with const base types is currently not supported in upstream BinaryMapping this template class is currently only enabled for non-const base types