Add a simple aggregate_kyoto_gases() method

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Description of PR

This PR adds a method to compute the aggregate Kyoto GHG from a set of species using any GWP metric supported by iam-units.

The method is similar to add_gwp100_kyoto() in the climate-assessment package.

One issue is that there is a discrepancy in the unit used for HFC43-10 between ScenarioMIP data, iam-units and common-definitions, so this is commented out for now. See iiasa/emissions_harmonization_historical#161

The units for each species and the minimum set of species (required) are hard-coded based on common-definitions, the returned variable has the GWP-metric as a suffix. This could be generalized in follow-up PR (or immediately if there are strong feelings).

Specific question to @jkikstra @znicholls @chrisroadmap:

• Is this the right set of species?
• Should the list of minimum/required species be extended?

[danielhuppmann]

This PR is a stepping stone to implement IAMconsortium/nomenclature#427

[znichollscr]

Is this the right set of species?
Should the list of minimum/required species be extended?

Really depends on the use case. For many cases, this looks totally fine (and is probably well over 95% of all Kyoto GHG emissions). For other cases, it will be missing fairly common gases. The 'full' list (according to simple climate models) is here: https://github.com/iiasa/emissions_harmonization_historical/blob/190affcf0caf71daeac395a45dd7c39846acbaec/notebooks/5194_post-process-emissions.py#L81. Making that required will probably drive most users crazy so I wouldn't recommend that. How many are optional really depends on use case I would say so hard to set in advance of knowing the use context.