Fixed missing inverted() fn.

Various code cleanups.

Use OpmResult defined in error module.

Separate Dottable trait to additional file.

Update docs. Update filter_test example.

Update documentation.

Various doc work

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

Add export function for OpticNode / Optical.

Fix some doc errors.

Add splitting ratio to Beamsplitter (not used yet).

Add example src-detector.