Velocity Controller-Relax Momentum Constraint

[CarlottaSartore]

This PR implements the hyperbolic tangent method to constraint the CoM position and removes the Linear Momentum Constraint
i.e.

$$ \dot{x}_{CoM,Lower}tanh(\epsilon_l[x_{CoM}-x_{lower}])<J_{CMM}\dot{s}_{K+1}<tanh(\epsilon_u [x_{upper}- x_{CoM}])\dot{x}_{CoM,upper} $$

where

• $x_{lower}$ is the lower bound of the support polygon;
• $x_{upper}$ is the upper bound of the support polygon;
• $x_{CoM}$ it the CoM Position computed via forward kinematic starting from $s$;
• $\dot{x}_{CoM,Lower}$ is the lower bound for the CoM Velocity;
• $\dot{x}_{CoM,Upper}$ is the upper bound for the CoM Velocity

In addition, minor fixes are introduced

C.C. @gabrielenava

[gabrielenava]

I reviewed the code and it seemed ok to me at a first glance. I did not run any controller which uses this library, therefore I could not debug for theoretical issues. But we're currently testing this controller offline with @CarlottaSartore, and it seems working fine. Therefore merging.