textdb: Add functions for cloning modified schemas, rows
spherical: Add fun to compute centroid of SphereCoords
AnalyzeResults: minor cleanup
textdb: New write_textdb taking rows directly, old becomes 'write_textdb_values'
ParseTweets: Add --grouped-location-method=centroid
ParseTweets: Handle timestamp on input, synthesize missing fields, don't warn multiple times for same field
spherical: Add bounding_box_sw, bounding_box_ne
math: Add quantile_at, expand some fns to take Iterable
new AnalyzeLocationVariance, determine props of user tweet locs