Release v0.16.0

What's Changed

• Allow nomenclatureconfig to have region country only by @phackstock in #320
• Making aggregation compatible with pyam 2.1 by @phackstock in #322
• Change dimension parameter to singular in cli by @phackstock in #323
• Disallow external repo double stacking by @phackstock in #324

Full Changelog: v0.15.0...v0.16.0