Updated instance_id utility function

[jbusecke]

• Deprecation warning on utils.cmip6_dataset_id + reminder issue to remove in next major release
• Add documentation with official sources about ESGF id structure (include member_id/instance_id discussion). Check back into issues at pangeo-forge-esgf. Which repo should hold this info?
• Kwarg input that switches between printing missing facets as none (default) and dropping them from the combined string (this should then be used in combine_datasets for the new dict key generation.

[jbusecke]

Flake8 formatting issue was solved with this. Adding r"..." did the trick

[jbusecke]

I need to stash this for later. Last action was to put a blank raise in the deprecated iid function to facilitate refactoring.