Tango shutter is_closed property could be smarter

Isabelle Kieffer wants to use is_closed property of Tango shutters but find it to be limited because Tango shutter can have more states than CLOSED to indicate the shutter is actually closed.

For instance when the door of the hutch is open, the shutter is obviously closed but is_closed returns False.

FrontEnd Example on ID21 during a MDT:

CYRIL [1]: fe._control
  Out [1]: DeviceProxy(acs.esrf.fr:10000/fe/master/id21,139998361132560)

CYRIL [2]: fe._control.state()
  Out [2]: tango._tango.DevState.STANDBY

CYRIL [3]: fe.is_closed
  Out [3]: False

CYRIL [4]: fe.is_open
  Out [4]: False