Vec operators mean() and cross() throw 'is not a function'

Hi, sorry it is me again.
Thanks for fixing the [Mat.mean() issue](https://github.com/justadudewhohacks/opencv4nodejs/issues/267). Now I wanted to use the vec mean() operator and got the following error:
```
Uncaught Exception:
TypeError: mat.mean(...).mean is not a function
```
I then tested out all the other vec operators and found that `cross()` also throws the same exception.
All other operators worked fine. Am I doing something wrong?
First I was calculating `mat.mean()` which gave me `Vec4 { z: 0, y: 0, x: 0, w: 0 }`
The result I then used for the vec operator like so `mat.mean().mean()`.