don't run if control signal is false
add is_blocked() for Elements and Connectors

Add a testcase where the control signal is "blocked" & extend a few
existing testcases to check if the processing element responds correctly
to is_blocked().
Add the main logic in Element : run_or_block(), is_blocked(), block()
Add a debug_state() function to show information about the state of all
elements & connectors in the flow
updated TODO list