Metadata: generate metadata sidecar files for invest model outputs

[davemfish]

This will be a new feature, to be merged into a feature branch.

The plan is to use geometamaker to generate YAML sidecar files for each invest model output file that is described in a model's MODEL_SPEC.outputs. https://github.com/natcap/geometamaker

Requirements:

• write a metadata file for each invest output file
• allow Workbench users to configure a default metadata profile with their contact info and license info, if they wish. See geometamaker.config.
• a new dictionary of keywords associated with models and/or output datasets, to be used in metadata
• release geometamaker
• make geometamaker compatible with Python 3.9

This is an objective of the NASA Metadata grant.