code: def write_src(hdf5_out, gctoo_object, out_file_name):
      
      if gctoo_object.src == None:
          hdf5_out.attrs[src_attr] = out_file_name
      else:
          hdf5_out.attrs[src_attr] = gctoo_object.src
docs: Writes src as attribute of gctx out file. 
  
  	Input:
  		- hdf5_out (h5py): hdf5 file to write to 
  		- gctoo_object (GCToo): GCToo instance to be written to .gctx
  		- out_file_name (str): name of hdf5 out file.