Version 0.5.0

• Add read and write functionality for interchange format
• Add csv reading capabilities in pm2io module for wide and long (tidy) CSV files.
• Add da.pr.coverage() and ds.pr.coverage() functions to summarize data
  coverage.
• Add aggregation functions set and count which use aliasing and can reduce to
  a given set of dimensions, including the entity.
• Update python packaging to use declarative style and modern setuptools.
• Support and test python 3.9 and windows.
• Add dataset attr for storing the terminology used for entities (and thus variable
  names).