clear DataFlowInterface in Service::clear()

This pull request adds the missing `DataFlowInterface::clear()` call to `Service::clear()` and updates the documentation of this function.

Note that `DataFlowInterface::clear()` only removes the ports from the service and does not automatically disconnect. The port instances might already have been destroyed (dangling pointers) if `clear()` is called from the service's destructor! There is no way to check if the ports still exists or if they are connected...