Messaging Improvements: Self-Dispatch, UNNAMED_ONLY

This MR

• Prevents modules to dispatch messages to themselves (more of a cosmetic fix since their execution is already over)
• Adds a new flag MsgFlags::UNNAMED_ONLY which allow to only listen to messages without specific name. One use case for this is a relay module that would e.g take unnamed MCParticle messages and re-route them to a module with input parameter set (like a DetectorHistogrammer)
• Some documentation updates.

This fixes #167 (closed)