make calibration interface uniform, matrix --> K

gtsam/geometry/Cal3_S2.h

@@ -125,10 +125,15 @@ public:
   }
 
   /// return calibration matrix K
-  Matrix matrix() const {
+  Matrix K() const {
     return Matrix_(3, 3, fx_, s_, u0_, 0.0, fy_, v0_, 0.0, 0.0, 1.0);
   }
 
+  /** @deprecated The following function has been deprecated, use K above */
+  Matrix matrix() const {
+    return K();
+  }
+
   /// return inverted calibration matrix inv(K)
   Matrix matrix_inverse() const {
     const double fxy = fx_ * fy_, sv0 = s_ * v0_, fyu0 = fy_ * u0_;