vtkSplineFilter: Add OutputPointsPrecion parameter

A parameter called OutputPointsPrecision is added to let the user choose
if vtkPoints generated by the vtkSplineFilter class is Single or Double
precision.

Previous implementation generated Single precision without any way to
modify it.

New implementation is done exaclty like vtkClipPolyData class :

  - Attribut OutputPointsPrecision is defined as protected member in
    vtkSplineFilter.h

  - Related vtkSetMacro and vtkGetMacro are added as public members in
    vtkSplineFilter.h

  - Default value is set in constructor

  - Precision is modified in RequestData method between points instanciation
    and points allocation by calling SetDataType method