Release v2.2.0

• Added: audbackend.checksum()
  for calculating MD5 sum of files.
  For parquet files
  it considers the "hash" metadata entry instead,
  if it is present
• Changed: retry to re-establish connection to backend two times
  before finally failing
• Removed: deprecated functions
  audbackend.access(),
  audbackend.create(),
  audbackend.delete(),
  audbackend.register()
• Fixed: storing of checksum
  on S3/MinIO backends