Update & extend unit tests accordingly.

Various code cleanups.

Use OpmResult defined in error module.

This cleans up the example code a little bit.
Fixed detector.export_data.
Generate diagram before performing analysis. If analysis fails,
a diagram is generated anyway.
Update various examples to use the aboe save function.

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.