R split measures the agreement between the sets of intensities created by merging 
              odd- and even-numbered images  from the overall data.          
	      Ref: T. A. White, R. A. Kirian, A. V. Martin, A. Aquila, K. Nass, A. Barty 
              and H. N. Chapman (2012), J. Appl. Cryst. 45, 335-341