Start a sumo server using cmd, establish a connection to it and
    store it under the given label. This method is not thread-safe.

def start(cmd, port=None, numRetries=10, label="default"):
    """
    Start a sumo server using cmd, establish a connection to it and
    store it under the given label. This method is not thread-safe.
    """
    if port is None:
        port = sumolib.miscutils.getFreeSocketPort()
    sumoProcess = subprocess.Popen(cmd + ["--remote-port", str(port)])
    _connections[label] = connect(port, numRetries, "localhost", sumoProcess)
    switch(label)
    return getVersion()