Rework delay between command: instead of sending null characters, which doesn't work on some chip sets, send commands from the _loop() function with a 100ms delay between consecutive commands.
The DFPlayer device does not like successive commands arriving to quickly after one another and may ignore a command. The driver has been modified to enforce a delay between commands by sending pad characters where necessary.
Performance enhancements in IODevice::loop() function.
Improved error handling, device is placed off line if not responding.
Improved error reporting, device shown as offline if not operational (faulty or not present).
When writing to analogue outputs pins, the digital _read() function now returns the 'busy' status of the analogue pin. Consequently, the _isBusy() function becomes superfluous and has been removed. The static IODevice::isBusy() function now calls the object's _read() function instead.
Also, limit in DFPlayer of 3 pins has been removed.
