Added BezierCurve handling and tests + some cleaning

src/spline/API/BezierCurve.h

src/spline/API/CubicFunction.h

src/spline/API/Curve_ABC.h

src/spline/API/ExactCubic.h

src/spline/API/MathDefs.h

src/spline/API/SplineVisitor.h

src/spline/BezierCurve.cpp

src/spline/CubicFunction.cpp

src/spline/ExactCubic.cpp

src/tests/spline_test/Main.cpp