Added option single to Instance.asdict() allowing to saving in multi-entity format

[jesper-friis]

Description

Closes #651

This PR adds the option single to Instance.asdict(). If you set single=False the dict-representation will be in multi-entity format, which is the expected format for data instances.

The default is currently set to True to retain the old behaviour. Ideally the default should be None, which would (like the json plugin) save metadata in single-entity format and data instances in multi-entity format.

If we change the default to None, a set of tests will currently fail. Question: should we keep the old behaviour, or spend the time to update all failing tests?

Type of change

• Bug fix & code cleanup
• New feature
• Documentation update
• Test update

Checklist for the reviewer

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?

[francescalb]

I think we should update the failing tests. I can give it a go.