Returns the skew matrix that is associated with a vector
skew(x)
Returns the 3 x 3 skew symmetric matrix associated with a 3-vector, i.e.,
cross(x,y) = skew(x)*y; skew(x) = [ 0 , -x[3], x[2]; x[3], 0 , -x[1]; -x[2], x[1], 0 ];