Pythonic improvements

[pkalita-lbl]

These changes address some minor performance and maintainability concerns.

• Move duplicated code in GeoEngine into private static methods.
• Use a @cache'd function to read CSV files so that they aren't read more than once and provide them as @property's on GeoEngine
• Rename various variables to follow standard naming conventions
• Use funcional-style testing
• Allow importing GeoEngine from top-level nmdc_geoloc_tools package