Add indexed_by= arg to Scenario.par_list() etc.

[khaeru]

message_data has a utility method iter_parameters("node") that returns the names of all ixmp parameters indexed by (for example) the set "node".

• Add to ixmp.Scenario.par_list() an optional argument indexed_by= that provides this feature.
• Enhance Scenario.set_list() etc. in the same way.
• (Extra) Add a more generic method like Scenario.list_items() that returns items of type set, par, equ, and/or var according to an argument.

[LauWien]

The utility method iter_parameters() got moved upstream from message_data to message-ix-models by commit iiasa/message-ix-models@4d35702.