separate radius for edges/nodes

[zsln]

This change allows to set a separate radius for edges and nodes without the need to create scalar quantities.
The functionality already exists via setRadius() but only for identical node/edge radius.

The reason for that is, that most of the times I don't want to have varying radii per element, but a constant but separate node and edge radius as a simple visual indicator of each segment of curve segments, i.e. to see if two edges only cross visually or if they are actually connected.
Using the new convenience functions setEdgeRadius()/setNodeRadius() allows for this without having to create scalar quantities and enabling them.

The changes:

• one additional PersistentValue per curve network
• setRadius() still exists and modifies both node/edge radius to preserve old behavior
• setEdgeRadius()/setNodeRadius() for modyfing all node/edge radii
• getRadius() is now split into getEdgeRadius()/getNodeRadius() is the only thing that breaks
• computeNodeRadiusMultiplierUniform()/computeEdgeRadiusMultiplierUniform() call the respective getEdgeRadius()/getNodeRadius() method