Metadata: design how Python users can generate metadata

[davemfish]

The metadata feature so far has focused on the Workbench & CLI users. Python API users might want to generate metadata too.

• Should it happen within execute? maybe as an option?
• Should there be a more prominent API for it? Right now we have natcap.invest.spec_utils.generate_metadata which has this signature:

def generate_metadata(model_module, args_dict):
    """Create metadata for all items in an invest model output workspace.

    Args:
        model_module (object) - the natcap.invest module containing
            the MODEL_SPEC attribute
        args_dict (dict) - the arguments dictionary passed to the
            model's ``execute`` function.

    Returns:
        None

    """

• Python API users could also use the geometamaker library directly, but the benefit of having another interface in natcap.invest is to take advantage of data from a MODEL_SPEC in invest.