Describe usecase: Fusing sensor outputs

[max-rosin]

Add content to ./doc/usecases/fusing_sensor_outputs.adoc.

• Add a description of the use case. Why would you want to do this? What options do you have? What are the most important considerations?
• Add prerequistes of this use case. What needs to be done before you can start this use case.
• Add relevant messages including a short description of how they are relevant to this use case.
• Add one or two examples.