Split an existing H2O data set according to user-specified ratios. The number of
subsets is always 1 more than the number of given ratios. Note that this does not give
an exact split. H2O is designed to be efficient on big data using a probabilistic
splitting method rather than an exact split. For example, when specifying a split of
0.75/0.25, H2O will produce a test/train split with an expected value of 0.75/0.25
rather than exactly 0.75/0.25. On small datasets, the sizes of the resulting splits
will deviate from the expected value more than on big data, where they will be very
close to exact.