Method for deduplicating supplemental data sets

When importing a dataset of e.g. NaviLens-enabled bus stops, some of those data points will be redundant to bus stops already marked in OpenStreetMap. The proper way to deduplicate this is probably to define some distance threshold, e.g. does a bus stop within 5m already exist in the database. If true, add the proper tags (e.g. navilens=true) to the existing point, rather than creating a new record in non_osm_data.

As flagged in #144.