New `check_encoder` config parameter for Axis object

When check_encoder is set to True, at the end of motion it compares encoder position with motor controller position ; if the two positions differ more than the allowed tolerance, an exception is raised. By default check_encoder is False.

The encoder_steps_per_unit property has been removed - to get encoder steps size, one has to declare an encoder and call axis.encoder.steps_per_unit.