Support for mono / single channel signal

Hello 👋,

To my understanding, the library currently only supports stereo signals as per [signature of `forward`](https://github.com/phudtran/rustft/blob/main/rustft/src/lib.rs#L71) :

```
pub fn forward(&self, input: ArrayView2<T>) -> Result<Array3<Complex<T>>, String>
```

Do you plan to also support audio signals with a single channel aka mono signals?