Filter all features in a collection by retaining only those that
    fall within the specified radius.

def features_keep_within_radius(obj, center, radius, units):
    """
    Filter all features in a collection by retaining only those that
    fall within the specified radius.
    """
    features_keep = []
    for feature in tqdm(obj['features']):
        if all([getattr(geopy.distance.vincenty((lat,lon), center), units) < radius for (lon,lat) in geojson.utils.coords(feature)]):
            features_keep.append(feature)
    obj['features'] = features_keep
    return obj