public Iterator<SubspaceIterator> subspaceIterator(Set<String> dimensions, DimensionSizes sizes) {
        return new SuperspaceIterator(dimensions, sizes);
    }
Returns an iterator over value iterators where the outer iterator is over each unique value of the dimensions
given and the inner iterator is over each unique value of the rest of the dimensions, in the
<i>standard value order</i>

@param dimensions the names of the dimensions of the superspace
@param sizes the size of each dimension in the space we are returning values for, containing
             one value per dimension of this tensor (in order). Each size may be the same or smaller
             than the corresponding size of this tensor
vespajlib/src/main/java/com/yahoo/tensor/IndexedTensor.java