forwards a message from an input queue to an output port, or to and from local
message queues. After a chart forwards a message, it can remove another message from
the queue in the same time step.
Checks the input queue for message
M_in. If a message
is present, the chart removes the message from the queue and forwards it to
the output port
M_out. After the chart forwards the
message, the message is no longer valid.
on M_in: forward(M_in,M_out);