[Requirement] Constraints on signals
- unicity of signals (internal to one external system, and across all external systems): naming + polarity
- all signals have a type
(from redmine: issue id 2721, created on 2016-10-18, closed on 2017-01-30)
(from redmine: issue id 2721, created on 2016-10-18, closed on 2017-01-30)