Add back in Optional to units

[coretl]

I may have been to hasty with #313, it would appear that original ophyd makes units=None in the descriptor:

Failed validating 'type' in schema['properties']['configuration']['additionalProperties']['properties']['data_keys']['additionalProperties']['properties']['units']:
    {'title': 'Units',
     'description': 'Engineering units of the value',
     'type': 'string'}

On instance['configuration']['det_old']['data_keys']['det_old_mode']['units']:
    None

We should add back in Optional, just for the units field (precision seems to be fine).