QUAT2DCM Conversion of a quaternion to an orthogonal rotation matrix.
 Assumes that the scalar element, q_w, is the first element of the 
 quaternion vector, q = [q_w q_x q_y q_z].
 
    R = quat2rotMat( q )
 
    If q needs to be a [4x1] row vector. R is a [3x3] SO3 rotation 
    matrix. You can batch process N quaternions by passing in a [Nx4] 
    matrix where each row is a quaternion. In this case thefunction 
    will return N rotation matrices of size [3x3xN].