Add external contact detection in motorFrictionIdentification

- compute gravity torques using WBI
- compute torques due to external contacts as difference between gravity
torques and measured torques
- if external torque is greater than threshold AND joint velocity is
lower than threshold, THEN use sample to estimate motor drive gain (i.e.
k_tau)
- plot external torque rather than torque derivative
- plot identification phase to understand what is being identified at
each moment