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The MySliderEventHandler is missing the m_callback whereas the SliderParams contains the m_callback field. Therefore, the m_callback is never called if the slider position is changed. This commit adds the missing m_callback field and call in SliderMove().
Fixes#613.
need to add streaming because memory is too small to store all lines
initial test of PD control in physics server, need to switch to PD control for motor constraint, instead of using external forces.
