R split measures the agreement between the sets of intensities created by merging
odd- and even-numbered images from the data within the resolution shell.
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