Doc work.
Remove no unsed function in Light.

Remove some debug meassages. Improve spectrum plot, code cleanups.

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

Fix documentation code example.

Add export function for OpticNode / Optical.

Fix some doc errors.

PaDoublepass example updated accrodingly.

Fix failing unit test.

For using reference nodes.

Show ports in dot file.

Not tested yet...

connect_nodes correctly sets the ports in Light structure.
Further checks necessary.

Add src & target port to add_nodes function. Update examples accordingly.
Improve OpossumError.