Add serde_yaml crate and add Serialize to spectrum.

Doc work.
Remove no unsed function in Light.

Add beam splitter to group. Analysis is still Ok.
Graph display still wrong.

Analysis seems to work. Display of graph wrong.

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

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.