Add job-name to generate_optimized_molecules

[JoshuaMeyers]

Our guacamol scoring functions do not have a name as part of the object. Its also not straightforward to add a 'name' to the various ScoringFunction's.

However each scoring function is named e.g. Celecoxib rediscovery. So this PR allows that name to be input (optionally) to the generate_optimized_molecules function.

This allows us to write intermediate files from generators, that have knowledge of the scoring function name.

Backwards compatibility is preserved!