[Proj] Distance calculation

[Karl Swartz]

> Zhang Xue-Lian (1985) The nested coefficient method for accurate 
> solutions of direct and inverse geodetic problems with any length. 
> Proceedings of the 7th International Symposium on Geodetic 
> Computations,  Direct and inverse distance 
> functions.              Cracow, June 18-21, 1985. Institute of 
> Geodesy and Cartography. Wasaw, Poland, ul. Jasna 2/4 pp 747-763
> 
> since it's more accurate than the classical method due to Gauss for 
> long distances.

How does this compare to Thomas Vincenty's algorithm (which can be found
at http://www.ngs.noaa.gov/PUBS_LIB/inverse.pdf)?

 -- Karl