Adding quadruped gaits

In this PR request is formulated different quadrupedal gaits (walking, trotting, pacing and bounding) plus a little jump with the quadruped.

For doing so, I significantly improve the SimpleQuadrupedalGaitProblem problem class. For instance, the foot swing trajectories used a step height.

These is the list of modifications:

• Defining a triangle swing trajectory based on the pre-define step height.
• Defining a CoM task which is needed for different gaits.
• Adding trotting, pacing and bounding gaits.
• Adding a jumping gaits (it needs to be improved).
• Easily definition of sequence of gaits.