@Override
        public void next() {
            int iterateDimensionsIndex = 0;
            while ( indexes[iterateDimensions.get(iterateDimensionsIndex)] + 1 == dimensionSizes().size(iterateDimensions.get(iterateDimensionsIndex))) {
                indexes[iterateDimensions.get(iterateDimensionsIndex)] = 0; // carry over
                iterateDimensionsIndex++;
            }
            indexes[iterateDimensions.get(iterateDimensionsIndex)]++;
        }
Advances this to the next cell in the standard indexed tensor cell order.
The first call to this will put it at the first position.

@throws RuntimeException if this is called when hasNext returns false
vespajlib/src/main/java/com/yahoo/tensor/IndexedTensor.java