Support for absolute encoders

[ceesietopc]

Some of the Dynamixels supported by the dynamixel_motor package have absolute encoders built in, e.g. the MX-28 and MX-106. This package however relies on the configuration of an initial position through a yaml file for the interpretation of current and goal positions. It would be nice if, for the models that support this, the absolute measurement was used.

(Great package, @arebgun! Worked like a charm on Kinetic on first configuration.)

[ceesietopc]

Upon further inspection, it seems the current position is the absolute position, my bad. In that case, the extend of this issue reduces: it would be nice if the init flag can be omitted for models that support it. It seems the init flag shifts the output of current_pos and thus the interpretation of goal_pos, which is not something that is expected.