Make some functions private.

If a port mapping was defined and group noed connected afterwards,
port maps might become invalid.

Check now if input / output nodes are half-connected.
Add unit tests accordingly.

To be used for improved port mapping.

Not yet tested at all.

Various doc work

This simply forwards incoming LightResult.
Rename RefNode::new -> RefNode::from_node to fix clippy warnings.

Function was completely rewritten. Fixed many clippy warnings and
reformatted code.

Works in principle but awfully slow because of resampling.

not yet tested.

Add export function for OpticNode / Optical.

Not working yet....

Change some function privacy settings.

Fix some doc errors.

Add splitting ratio to Beamsplitter (not used yet).

Add example src-detector.