@Override
        public Tensor.Builder cell(TensorAddress address, double value) {
            TensorAddress sparsePart = address.mappedPartialAddress(index.sparseType, index.type.dimensions());
            int denseOffset = index.denseOffsetOf(address);
            double[] denseSubspace = denseSubspace(sparsePart);
            denseSubspace[denseOffset] = value;
            return this;
        }
For each sparse partial address, hold a dense subspace
vespajlib/src/main/java/com/yahoo/tensor/MixedTensor.java