Landmark initialization

We need a better way to initialize a new landmark. This issue proposes the following algorithm

1. Check for pre-conditions:
   • feature is not associated to any lmk
   • feature track is long enough
   • feature track spans at least two keyframes
2. Given 2 keyframes, first and last in track:
   • Recover 2 cv::keypoints pi
   • Compute 2 projection matrices Pi = [K 0] [Ri Ti ; 0 1]
   • Compute lmk point x = cv::triangulatePoints(std::vector(pi), std::vector(Pi))
3. Given N keyframes in track: for each i=1..N
   • Compute errors ei = pi - pinhole(Ti, qi, x)
   • Compute Jacobians Ji = dei/dx
   • Compute hessian H = sum Ji.tr * Ji
   • Compute rhs vector b = sum Ji.tr ei
4. check numerical conditioning of H. If pass:
   • Compute dx = - H.inv * b
   • Update x = x + dx
   • Initialize landmark at position x