Sampling frequency in Fresnel Propagator with ZoomFFT

Sampling frequency in the `_zoomFourierTransform2D` method is calculated as
```
sample_frequency_x = len(x) / x_range
```

This should be 
```
sample_frequency_x = (len(x)-1) / x_range
```
and similarly for the y-axis

Otherwise introduces a phase tilt. 