Add support for partial data format to Optimade python tools.

[JPBergsma]

After the last optimade workshop, the option to return data in parts was added to the OPTIMADE specification. OPTIMADE PR#467
With this issue, I would like to keep track of the different steps needed to implement this in the optimade-python-tools.

• Create a pydantic model of the partial data response format.
• Add code to convert python object to appropriate jsonlines response.
• Add validators
• Add unit tests for validator model and jsonlines response generator.
• Add example data
• Add code to recognize a property should be returned as partial data and correctly split up data in this case.
• Add/Update integration tests

Questions ?

It would seem logical to store data/files in Grid FS of MongoDB. Do we want to enable this by default or make it optional ?