Better promotion of correctly exporting a DataCube (or related) to valid JSON process representation

[soxofaan]

Related to, triggered by: #155, #209, Open-EO/openeo-web-editor#271

When users are asked to share a process graph they should use DataCube.to_json instead of DataCube.flat_graph, DataCube.flatten or DataCube.graph:

• the shared structure should have a toplevel {"process_graph": ...}
• it should be serialized as valid JSON (double quotes, booleans true/false), not python dict representation (single quotes by default, booleans True/False)

Possible action points:

• improve documentation: better promote correct way, more strongly discourage the wrong ways
• is it possible to trigger warnings when normal users use the "flat_graph" methods (which are still used internally)?

[soxofaan]

Another idea: also add DataCube.print_json, which might be a more user friendly solution in a notebook context than DataCube.to_json (which requires an additional print call to make the result usable)

[soxofaan]

• removed the duplication of bad usage patterns: DataCube.graph and DataCube.flatten(), only DataCube.flat_graph() is kept, which is heavily used internally
• docs: stronger discouragement of using flat_graph() for general usage
• added print_json()
• added documentation section on how to properly export valid process (graph) JSON: https://open-eo.github.io/openeo-python-client/cookbook/tricks.html#process-graph-export

[soxofaan]

Will close this for now