Add GUA to geomagio/Metadata.py

GUA is transitioning to an ObsRIO data logger, which requires a metadata entry in Metadata.py. We do not know actual calibration values for the GUA Narod mag, and cannot retrieve them currently due to ongoing communication issues, so we'll use Narod default calibration parameters.

This commit applies strictly to GUA. Additional commits are forthcoming that relate to the test FGE+ObsRIO system that will also be deployed soon.