Resolve "Add outgoing port to detector node"

Closes #21

See merge request !2

Update documentation.

Resolve "Spectrum: Add serde serializer / deserializer"

Closes #23

See merge request !1

Tiny doc fixes.

Add serde_yaml crate and add Serialize to spectrum.

Doc work.
Remove no unsed function in Light.

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.

Add beam splitter to group. Analysis is still Ok.
Graph display still wrong.

Analysis seems to work. Display of graph wrong.