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 #1780 from iglesias/develop

Change directory contents to contents in README to fix section link
Add Dockerfile for trusted builds on index.docker.io
Merge pull request #1774 from tklein23/solabels_factory_in_somodel

Structured model refactoring: Added factory for specific labels
Fix assertion - open() can return "0", which is a valid file descriptor.
Merge pull request #1782 from tklein23/develop

Fix assertion - open() can return "0", which is a valid file descriptor.