Only write parent attributes when they are required for parent experiments in CMIP7 datasets

[mauzey1]

Resolves #937

Adds CMIP7-specific requirements for parent experiments (parent_experiment_id) and other parent attributes. These are enabled by setting "_cmip7_option": 1 in the user input JSON.

For experiments with no parent experiments defined in the CV, CMOR now warns on stray parent attributes and removes them from the dataset before writing. It also hard-errors when a user supplies parent_experiment_id for an experiment that should not have one, and when a required parent_experiment_id is missing for an experiment that does define parents.

Test/test_cmor_parent_attrs.py covers the following cases:

• CMIP7 output with no parent attributes for no-parent experiments.
• CMIP7 output with the full required parent attribute set for experiments like historical.
• Warning-and-removal behavior when stray parent attributes are present without a parent experiment.
• Erroring when a user defines parent_experiment_id for an experiment whose CV lists no parents.
• Erroring when a required parent_experiment_id is omitted.
• Erroring when the user supplies a parent experiment not allowed by the CV.
• Erroring when the user supplies an incorrect parent_activity_id.

@matthew-mizielinski

[taylor13]

The changes listed above seem sufficient to handle everything the way we want. I've looked at the code changes briefly and didn't notice any issues, but can't say that should be a consideration.