added to_dot_test

Add unit tests.

The original graph is kept untouched.

This is only a preliminary solution until we have a general reporting.

Copied from EnergyMeter. Needs modifications yet.

Update docs. Update filter_test example.

Update documentation.

Tiny doc fixes.

Add serde_yaml crate and add Serialize to spectrum.

Doc work.
Remove no unsed function in Light.

Make some functions private.