Put back pedometer_steps

[caternuson]

Possible fix for #21

Simply puts back the class level ROUnaryStruct register for steps. Looks like this was accidentally removed during a refactor in #16. Since that PR seems to be related to conserving memory, not sure if this is the desired solution. But it checks out:

Adafruit CircuitPython 5.3.1 on 2020-07-13; Adafruit CLUE nRF52840 Express with nRF52840
>>> import board
>>> from adafruit_lsm6ds import LSM6DS33
>>> sensor = LSM6DS33(board.I2C())
>>> sensor.pedometer_enable = True
>>> sensor.pedometer_steps
0
>>> sensor.pedometer_steps
10
>>> 

[siddacious]

Thanks @caternuson, however I noticed this and there is a fix in #20 . I'll work on getting it merged