Add a function to close the serial port

[olmerg]

The object to close the serial port is not accesible , os will be good have a function for close the port.

        function close_port(obj)
      % tells if msg is an ip address, returns the ip and port if so.
        if obj.connection_type == 'serial'
          fclose(obj.connection);
        endif
    endfunction

Maybe also is important to review is the port is open (during constructor) to generate this error an not
error: Invalid call to arduino. Correct usage is:

[Molorius]

Thank you for the suggestion, but I will have to decide how to close the port so it is still Matlab compatible.

I will have it display a better error message within the next few days.