Transfer PointSet from Moving Image to Fixed Image

Hi @thewtex @blezek @bradking @cpatrick @arnaudgelas @dzenanz @N-Dekker

In the example, [ITK_Example09_PointSetAndMaskTransformation.ipynb](https://github.com/InsightSoftwareConsortium/ITKElastix/blob/master/examples/ITK_Example09_PointSetAndMaskTransformation.ipynb), it is mentioned ...

When transforming a point set, this happens the other way around: from the fixed image domain to the moving image domain. This is useful for example when you want to know where a point maps to. Point sets therefor need not be transformed with the backwards mapping protocol, but can instead be transformed with regular forward transformation (fixed -> moving). Transforming point sets can be used to get the regions of interest (ROI) in the moving image, based on ROI of the fixed image.

**Can we transfer points in moving image to the fixed image domain ?**
How to do the backwards mapping in this case ?
What change should be made here ?
```
# Procedural interface of transformix filter
result_point_set = itk.transformix_pointset(
moving_image, result_transform_parameters,
fixed_point_set_file_name='data/CT_3D_lung_fixed_point_set_corrected.txt',
output_directory = './exampleoutput')
```

Thanks