Sample crystal structures with GGen for GenBench

GenBench's sample contract: num_samples GGen trials for a formula, or for stoichiometries drawn from a chemical system, each a random symmetric crystal relaxed with ORB v3 and returned as an inline CIF. Duplicates are kept and nothing is saved to the shared database or published, so a benchmark can call it many times. GGen needs a composition, so the unconditional task answers 422. To find the most stable structure of a formula, use Generate a crystal structure using GGen instead.