Validate multiple instances

[jesper-friis]

Description

Update dlite-validate to validate sources with multiple instances, without having to provide the --id option.

This PR build on top of PR #985.

Update: Added new Python interface to the jstore implemented in C. Right now this increases the code base, but this change makes it possible to e.g. reduce utils.instance_from_dict() and Instance.asdict() to a few lines of code. An added benefit would be consistency with JSON parsing (since the json parser in C is used under the hood).

Suggests to take the code removal/replacement in a separate PR to have little more control over changes.

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?