Missing metadata for res and den outputs

[rwblair]

Derivatives specification says if the res entitiy is used Resolution must be defined in its json file:
https://bids-specification.readthedocs.io/en/stable/05-derivatives/03-imaging.html#preprocessed-coregistered-andor-resampled-volumes

The actual content of the Resolution field is pretty wide open so boiler text explaining the number directly corresponds to millimeter resolution should be good enough.

This is an issue for both anatomical and functional outputs.

[mgxd]

• Ensuring Resolution metadata in outputs with res entities (ENH: Add Resolution field to metadata if data has res entity #754)
• Ensuring Density metadata in outputs with den entities