The node RealLens is missing in the repo..

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

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

Works in principle but awfully slow because of resampling.

not yet tested.

Update example accordingly.

no unit tests yet.

Add boilerplate "filter" function.

Fix some doc errors.

Add splitting ratio to Beamsplitter (not used yet).