better tolerance for plane

Geo/GFace.cpp

@@ -450,7 +450,7 @@ void GFace::computeMeanPlane()
       for(unsigned int i = 2; i < pts.size(); i++){
         SVector3 d0i(pts[0], pts[i]);
         SVector3 n = crossprod(d01, d0i);
-        if(norm(n) > 1e-12){
+        if(norm(n) > 1e-6){
           res[0] = n.x(); res[1] = n.y(); res[2] = n.z();
           xm = pts[0].x(); ym = pts[0].y(); zm = pts[0].z();
           ok = true;