Long overdue release

Much has changed over the last two years.

• Iris code has been moved to a separate directory to make room for other code (such as Xarray)
• Catalogue is now more generic and can work with data archives that use the CMIP directory strucutre and naming conventions (e.g., CMIP5, CMIP6, HAPPI).
• Examples and notebooks are now included
• The beginnings of statistical downscaling code is now available in _iris