Special handling of dict values, esp. by quantulum

[turbomam]

Could this library accept dictionary values? Consider when an upstream process is already generating JSON that isolates the units from a measurement, but doesn't do any other sanitization

        "depth": {
            "has_raw_value": "0-1",
            "has_unit": "m"
        },
        "elev": {
            "has_raw_value": 500.0,
            "has_unit": "m"
        }

Right now, sample-annotator is complaining (on quantulum's behalf) that depth and elev are dicts, not strings.