Method for selecting half datasets used in computing Rsplit,
CC1/2 and CCstar.
The following enumerated values are used:
by_observation: unmerged reflection intensities are randomly
divided into two half-sets of nearly equal size. As recommended
in Karplus PA, Diederichs K. Linking crystallographic model and
data quality. Science. 2012;336(6084):1030-1033.
by_lattice: often used in serial crystallography, crystals are
pre-sorted into two half datasets of nearly equal size (such as
by odd vs. even crystal number).