Convert skew-symmetric matrix to vector
V = vex(S)
V = VEX(S) is the vector (3x1) which has the skew-symmetric matrix S (3x3)
This is the inverse of the function SKEW().
No checking is done to ensure that the matrix is actually skew-symmetric.
The function takes the mean of the two elements that correspond to each unique element of the matrix, ie. vx = 0.5*(S(3,2)-S(2,3)).