@Override
        int nextDimensionsAtStart() {
            int dimension = 0;
            while (dimension < iterateDimensions.size()  && indexes[iterateDimensions.get(dimension)] == 0)
                dimension++;
            return dimension;
        }
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