(lat,lon)| returns the cumulative distance |d| in meters along the path 
% specified by geo coordinates |lat,lon|. Coordinates must be vectors of equal size. 
%
% |d = pathdistps(x,y)| returns the cumulative distance |d| in meters along the path 
% specified by polar stereographic coordinates |x,y| where |x| and |y| are vectors of equal
% size in ps71 meters.  
%
% |d = pathdistps(...,'km')| simply divides output by 1000 to give distance in kilometers. 
% 
% |d = pathdistps(...,'ref',[reflat reflon])| refer