[Proj] The area of a rhumb line polygon

[Charles Karney]

The calculation of the area of a geodesic polygon has been included in
GeographicLib since v1.4 (2010-09).

In the latest version v1.39, I've also included the equivalent
calculation when the edges of the polygon are rhumb lines.  The
formulation is given in

   http://geographiclib.sf.net/html/rhumb.html#rhumbarea

and the online planimeter

   http://geographiclib.sf.net/cgi-bin/Planimeter

implements this area calculation.  I've also worked out the area when
the edges are great ellipses.  For the formulation, see

   http://geographiclib.sf.net/html/greatellipse.html#gearea

An implementation is included in the Matlab package

   http://www.mathworks.com/matlabcentral/fileexchange/47898

Comments are welcome.

   --Charles