Creating ntuples

Function: gsl_ntuple * gsl_ntuple_create (char * filename, void * ntuple_data, size_t size)

This function creates a new write-only ntuple file filename for ntuples of size size and returns a pointer to the newly created ntuple struct. Any existing file with the same name is truncated to zero length and overwritten. 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.