distinguish riding and walking persons in fcd output

[namdre]

riding persons are written subsequently to the vehicle the are riding in. As long as another vehicles is written afterwards, the order of elements implies ridership. However, for persons that are written after the last vehicle there is ambiguity in regard to riding and walking

[namdre]

best solution so far:

• add option for configuring more optional attributes (i.e. fcd-output.attributes x,y,speed,vehicle) (allow specifying written attributes for various outputs #7632)
• optionally write vehicle for persons (empty string if not riding)