Backwards incompatible changes in the SAGA API

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Version 7
- class CSG_Parameter_Data has been merged into class CSG_Parameter
- CSG_Parameter_Range: Get/Set-Lo/Hi functions renamed to Get/Set-Min/Max
- CSG_Tool_Grid::Get_System() now returns reference instead of pointer
- CSG_Data_Manager::Add(file): returns csg_data_object pointer instead of boolean
- SG_Get_String(), precision parameter defaults to -99 (= standard)

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