Version 0.3.0

• Add functions for downscaling and aggregation of gas baskets and categorical baskets.
• Add functions, docs, and tests for basic GWP handling.
• Add Makefile target to generate patched stub files for xarray containing the primap2
  API.
• Add development documentation detailing the structure of the repository and the tools
  used to development of PRIMAP2.
• Add selection and indexing which understands dimension names like area in addition
  to the full dimension key including the category set like area (ISO3). Works with
  ds.pr[key] and ds.pr.loc[selection] as well as da.pr.loc[selection].
• Add usage documentation for all currently included functionality.
• Access metadata easily via properties like ds.pr.references.