TrivialSplits() identifies trivial splits (which separate one or zero
leaves from all others); WithoutTrivialSplits() removes them from a
Splits object.
TrivialSplits() returns a logical vector specifying whether each
split in splits is trivial, i.e. includes or excludes only a single tip or
no tips at all.
WithoutTrivialSplits() returns a Splits object with trivial
splits removed.