Enable function of ImprintGeometry operation.

Uncomment code that actually calculated distance.

Set number of threads for vtkMultiThreader, so code actually runs on multiple threads.

Use vtkImageData::TransformIndexToPhysicalPoint() so image coords are properly located in world space.

Add 'Imprint Distance' option to allow seeing the distance field, instead of just the presence/absence of the input mesh.

Fix aeva#255 (closed)