'Sketcher_Utils::MakeInterpolation' were changed to interpolate each spline
section by the cubic B-spline passing through the control points that the
tangent vector in each control point P is calculated by the following way:
- if point P is preceded by a control point A and is followed by a control point
B then the tangent vector is equal to (P - A) / |P - A| + (B - P) / |B - P|;
- if point P is preceded by a control point A but is not followed by any control
point then the tangent vector is equal to P - A;
- if point P is followed by a control point B but is not preceded by any control
point then the tangent vector is equal to B - P.
