LongitudinalDistanceAction timeGap and distance are mutually exclusive but stereotyped as attributes

Describe the bug

In LongitudinalDistanceAction we express timeGap and distance fields as attributes and hence cannot enforce they are mutually exclusive, only documenting so in their descriptions.

Describe the expected behavior

These fields are converted to child subtypes that allow a choice element to mutually exclude them in the XSD schema.

It would probably be good to keep LateralDistanceAction.distance consistent.