Version 3.0.0 Release Candidate 2

Version 3.0.0 RC2

Minor changes to demography to have consistent attribute naming and to allow 'empty' cohorts to be generated.

What's Changed

• Version 300rc2 changes by David Orme (@davidorme) in #696

Full Changelog: v3.0.0-rc.1...v3.0.0-rc.2