   returns:

      jacobi   the matrix of partial derivatives of the conversion from
               planetographic to rectangular coordinates evaluated at the
               input coordinates. This matrix has the form

               If [1,1] = size(lon) then [3,3]   = size(jacobi)
               If [1,n] = size(lon) then [3,3,n] = size(jacobi)
                                          double = class(jacobi)

                   -                              -
                  |  dx/dlon   dx/dlat   dx/dal