add new vec func

Geo/SVector3.h

@@ -91,6 +91,11 @@ class SVector3 {
   }
   const double* data() const {return P.data();}
   double* data() {return P.data();}
+  double axpy(const double a, const SVector3& y){
+    for (int i=0; i<3; i++){
+      P[i] += (a*y[i]);
+    }
+  }
 };
 
 inline double dot(const SVector3 &a, const SVector3 &b)