Add support for "small" arcs on ellipsoid

[dengwirda]

Currently, ellipsoidal-mesh geometry is defined in terms of "great ellipses". It'd be nice to add support for "small ellipse" arc definitions too, to support, e.g. arcs of constant latitude, etc.

Could do this (perhaps?) by introducing a new edge3 element type, where the 3rd node could be used to determine the plane the arc is inscribed on.