Change reachability API for quasiStatic methods

[pFernbach]

• Add optional argument to force the computation of only one intersection, even in the case of a contact repositionning (false by default to keep the previous behavior)
• Improve API of intersectionExist for the cost function : min distance to a specified point
• Use the center of the support polygon of the phase with the fewer contacts (with a z value equal to the average of the CoM height between the init and final state) as the point used by for cost function