[X1,X2]=VL_WITPS(XP1,XP2,Y,Yp) computes the inverse thin-plate spline
(TPS) warp of the points XP1,XP2.
Remark
The inverse of a thin-plate spline in general is NOT a
thin-plate spline and some splines do not have an inverse. This
function uses Gauss-Newton to compute a set of points (X1,X2)
such that [XP1,XP2]=VL_WTPS(X1,X2,Y,Yp).