Followup to !3091: Streamline new decay finder implementation

• do not export component specific headers to the rest of the world (esp. not with such generic names as 'Node' and 'Arrow')
• do not add unnecessary additional namespaces in global scope (i.e. instead of a separate 'finder' namespace just re-use the IDecayFinder scope)
• simplify the Arrow class to the point it is just an enum
• do not move from a const& -- pass by value and then move instead
• do not repeat member initializers in constructors when not needed
• prefer standalone functions over member functions that do not interact with the class they are a member of