added warning for homing offset on dxl

[aedsinger]

    Users may incorrectly set the homing offset on a single-turn servo
    In general on single-turn servos, the homing offset should be zero
    In some cases it may be non-zero but between -1024 and 1024 (per DXL spec)
    If it is outside of the valid range it is ignored. However,
    when the servo is placed in other control modes (eg vel) the reported encoder position start using the offset
    This can create unexpected behavior

This PR adds the above warning and checks for a bad homing offset.