add iD.geo.geo_dist for (approximate, relative) geographical distance measurments

iD.geo.dist returns the euclidian distance, and should therefore
only be used for calculations of projected coordinates or
display (pixel) coordinates.

This introduces a function for approximate geographical distances
that accounts for distortions at higher latitudes. This metric can
be used for determining the nearest node (operations.Delete,
actions.Circularize) or relative length comparisons (actions.Split).