Add addtional AddRaster nodes at runtime #207

[Chamlika]

Sample usage from within the user function.

    def update(self, data):
        base = data[
            "base"
        ] 
        dynamic_data = {
            "AR": [
                {
                    "data_path": "c:/temp",
                    "filter": "*",
                    "dataset_id":"007",
                    "raster_type":"Table / Raster Catalog"
                },
                {
                    "data_path": "c:/temp",
                    "filter": "*",
                    "dataset_id":"008",
                    "raster_type":"Raster Dataset"
                }
            ]
        }
        ret = base.add_raster_nodes(dynamic_data)
        return ret