Fix bug icubWholeBodyState::getEstimate

When reading a single joint pos/vel/acc estimate the indexing was wrong.
New algorithm to compute pwm offset

Rather than taking as pwm offset the value of PWM used by the position
controller to maintain the joint in the initial position, I move the
joint up and down to try to estimate the stiction and so have a pwm
offset that is not biased by the current joint stiction.