Resolve "Add detector node "Energy meter""

Closes #29

See merge request !3

Update docs. Update filter_test example.

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.