0.6.2.dev3

Changes in 0.6.2.dev3

• Introduced abstract base class xcube.util.jsonschema.JsonObject which
  is now the super class of many classes that have JSON object representations.
  In Jupyter notebooks, instances of such classes are automatically rendered
  as JSON trees.
• xcube gen2 CLI tool can now have multiple -v options, e.g. -vvv
  will now output detailed requests and responses.
• Added new Juypter notebooks in examples/notebooks/gen2
  for the data cube generators in the package xcube.core.gen2.