added to_dot_test

Tiny doc fixes.

Add serde_yaml crate and add Serialize to spectrum.

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.

Add export function for OpticNode / Optical.

Update example accordingly.

no unit tests yet.

All unit tests pass.

resample fn seems to work now.

works in pricniple but way too complicated..

Add boilerplate "filter" function.