Improve slice visualisation by using `vtkImageResliceMapper` instead of `vtkImageSliceMapper`

[paskino]

The mapper for the image slice displayed is an instance of vtkImageSliceMapper.

CILViewer/Wrappers/Python/ccpi/viewer/CILViewerBase.py

Lines 84 to 86 in e30cc76

	imageSlice = vtk.vtkImageSlice()
	imageSliceMapper = vtk.vtkImageSliceMapper()
	imageSlice.SetMapper(imageSliceMapper)

Looks like from this thread that we may get better image quality with vtkImageResliceMapper
https://discourse.vtk.org/t/qvtknativewidget-jagged-edges-on-2d-images/16188/4