Resolve "Not all losses are added to the defaults-list"

Closes #66 (closed)

This MR changes the naming conventions for files in hydra_models. They now have to be the same as the default group name that will be used in config files later. Before this MR we used plural for the file names but singular for imports. By changing this convention we can now fully automatically add all classes that were automatically generated. This means automatically all hydra_models data-classes are added to the config-store without a human having to import them in the add_hydra_models_to_config_store.py file.