Meaningful organizationalUnits elements for sub-schemas

It is already good, to support all organizationalUnits in the root railml/metadata element.

It would be better, to only provide meaningful organizationalUnits child elements for the sub-schema/metadata element. The "meaningful" organizationalUnits may be identified by all optional references within the sub-schemas. Currently, all child elements are possible.

<railml>
  <metadata>
    <organizationalUnits>
      <infrastructureManager.../>
      <vehicleManufacturer.../>
      <vehicleOperator.../>
      <customer.../>
      <railwayUndertaking.../>
      <operationalUndertaking.../>
      <concessionaire.../>
      <contractor.../>
    </organizationalUnits>
  </metadata>
  <infrastructure>
    <metadata>
      <organizationalUnits>
        <infrastructureManager.../>
        <contractor.../>
      </organizationalUnits>
    </metadata>
    ...
  </infrastructure>
  <rollingstock>
    <metadata>
      <organizationalUnits>
        <vehicleManufacturer.../>
        <vehicleOperator.../>
        <contractor.../>
      </organizationalUnits>
    </metadata>
    ...
  </rollingstock>
  <timetable>
    <metadata>
      <organizationalUnits>
        <vehicleOperator.../>
        <customer.../>
        <railwayUndertaking.../>
        <operationalUndertaking.../>
        <concessionaire.../>
        <contractor.../>
      </organizationalUnits>
    </metadata>
    ...
  </timetable>
</railml>