Added StructuredModel::structured_labels_factory() to create the right SO labels class.
Prepared StructuredLabels' methods for overloading.
Overloading add_label, set_label, get_label, get_num_elements in MulticlassSOLabels.
Added own storage object for real labels.
Temporarily disabled ensure_valid calls.
Fixed memory leak (forgot freeing of new CRealNumber labels).
Merge pull request #1774 from tklein23/solabels_factory_in_somodel

Structured model refactoring: Added factory for specific labels