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.

Fix documentation code example.

Add export function for OpticNode / Optical.

Not working yet....

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.