## Opening an existing ntuple file

Function: gsl_ntuple * gsl_ntuple_open (char * filename, void * ntuple_data, size_t size)
  This function opens an existing ntuple file filename for reading and returns a pointer to a corresponding ntuple struct. The ntuples in the file must have size size. A pointer to memory for the current ntuple row ntuple_data must be supplied -- this is used to copy ntuples in and out of the file.