Initializer for Vec::zeros()

Added `zeros()` initializer for `Vec` (Task #3380)
Passed all tests (`release/bin/opencv_test_*`)
Tested feature by:
```c++
main() {
  Vec3f a = Vec3f::zeros();
  cout << a << endl;
}
```

PS: I'm a noob, and this is my first PR. Feel free to pick nits. And will appreciate any guidance. Thanks!