While this is Ok in Linux, building the tests will fail in Visual Studio.

See: paraview/paraview#22392

ColorMapToLUT_JSON now reads JSON files with multiple color maps.

The JSON colormap parser can now read from JSON files containing more than one colormap. More fixes to the parsing of colormaps done.

`vtkImagingFactory` was removed in VTK version 6, but this example runs
just fine with any reference to that class removed.

Also using vtkTypeFloat64 instead of double.