BUG: Double the distance 'skeleton to shell' in the MedialThicknessImageFilter3D

[T4mmi]

Thickness should be the equivalent diameter and not radius (i.e. double the distance from the medial axis to the outter shell)

• Filter code was fixed (add MultiplyImageFilter to the pipeline)
• Behavior was explicited in the README.rst
• Output test data were adjusted to the new behavior

[T4mmi]

Thanks for the feedback, I'll integrate your remarks