Add a Graph.meta_data field

[PaulSchweizer]

For conversions of graphs to other systems, it would be beneficial to have a meta_data field on the graph to control the conversion.

[philnewm]

For now a meta_data field that just excepts arbitrary data in a dictionary or something like that would already work.
The key element would be to pass this kind of meta_data information onto a resulting job which gets created based on the graph.