Rounded edges in radial detector visualization

This MR introduces a new VisualizationGeant4 parameter called line_segments, which sets the number of lines that are used to approximate a circle in the visualization. This change affects only the visualization of radial strip detectors, which now correctly appear with rounded edges instead of being approximated with a straight edge.

Additionally, the transparency parameter of the VisualizationGeant4 was renamed to opacity as that was its actual effect (transparency=0 meant fully transparent, transparency=1 fully opaque). The parameter newly cannot be set to zero, which was previously possible and caused that nothing was being visualized and Geant4 got mad when trying to display a blank scene.