Do NOT open silently on read/write when closed

This was a great idea: if the device was closed, due to a read/write
error, open it on read or write.  However, the device is then not
initialized.  It should be the task of the MuonLab class to reinitialize
the device (set thresholds and voltages, for example).
Raise ClosedDeviceError if device is closed
Muonlab app: reopen if closed, use old settings when appending