The original graph is kept untouched.

Make some functions private.

If a port mapping was defined and group noed connected afterwards,
port maps might become invalid.

Check now if input / output nodes are half-connected.
Add unit tests accordingly.

To be used for improved port mapping.

Not yet tested at all.

Various doc work

Fix some doc errors.

Add splitting ratio to Beamsplitter (not used yet).

Add example src-detector.