Multi_ts API example adding, available_operations from API bug fixed
- available_operations for composers which were set by user were ignored and changed on preset, fixed by check operations before using preset;
- single_drop_mutation could delete all nodes from graph and falls with exeption "IndexError: Cannot choose from an empty sequence" in nex call. Fixed by check if graph.nodes list empty at mutation applying step;
- multi_ts data type usage via API example fixed and added to repository.