This function will recalculate the spline parameters and store them in _data, ready for spline interpolation

=========================================================================== GENERATE QUADRANGLES =====================================================

* \brief Lifts a point from the image plane to its projective ray * * \param p image coordinates * \param P coordinates of the projective ray */

* \brief Lifts a point from the image plane to its projective ray * * \param p image coordinates * \param P coordinates of the projective ray */

* \brief Lifts a point from the image plane to its projective ray * * \param p image coordinates * \param P coordinates of the projective ray */

* \brief Lifts a point from the image plane to the unit sphere * * \param p image coordinates * \param P coordinates of the point on the sphere */

* \brief Lifts a point from the image plane to the unit sphere * * \param p image coordinates * \param P coordinates of the point on the sphere */

* \brief Lifts a point from the image plane to the unit sphere * * \param p image coordinates * \param P coordinates of the point on the sphere */

* \brief Lifts a point from the image plane to the unit sphere * * \param p image coordinates * \param P coordinates of the point on the sphere */

Check if the spline has been calculated, then generate the spline interpolated value