Version 3.1.0

• Increased version number to 3.1 because the Cate Web API changed in an
  incompatible way. Dataset descriptors now have a data_type
  field instead of a type_specifier field in 3.0.x.
• The --auto-stop-after <seconds> option now works correctly with
  Cate service CLI cate-webapi-start command. It will be used to
  automatically stop inactivate Cate Cloud Service instances.
• Fixed bug that would cause that data that was downloaded and cached locally
  could not be opened.
• Increased xcube and xcube-cci version to >=0.9.1 and fixed issues
  that arose from that change:
  • Type specifiers replaced with data types (DATASET_TYPE) and removed
    TYPE_SPECIFIER_CUBE and TYPE_SPECIFIER_DATASET
  • get_type_specifiers_for_data replaced with get_data_types_for_data
  • Changed xcube directory configurations to file.
• Changed Dockerfile. Is now completely based on quay.io/bcdev/xcube.
• Unpinned xarray version (now 0.19.0+), unpinned pandas version (now 1.3+)