CombinedFilter lifts a Sequence of GeneratorFilter[T] filters into a single GeneratorFilter (which filters a vector, here reified as a metrics.
A utility class, so we only have to extend the 'passesFilter' method in subclasses.
Used by FindReads, a GeneratorFilter is a predicate on values, which also wraps a particular BucketComparisons object (the thing that produces those values).