Function for low power mode

It looks like the [Adafruit_BNO055 C++ library](https://github.com/adafruit/Adafruit_BNO055/blob/1278d9caa933ac4ff9ea4780b4e9a0f02bbc3cef/Adafruit_BNO055.cpp#L791-L816) has `enterSuspendMode`.  This would be nice as a method inside the python library