void addBlock(DenseSubspace block) {
                if (block.cells.length != index.denseSubspaceSize) {
                    throw new IllegalStateException("dense subspace size mismatch, expected " + index.denseSubspaceSize
                            + " cells, but got: " + block.cells.length);
                }
                builder.put(block.sparseAddress, count++);
                listBuilder.add(block);
            }
An immutable index into a list of cells.
Contains additional information required
for handling mixed tensor addresses.
Assumes indexed dimensions are bound.
vespajlib/src/main/java/com/yahoo/tensor/MixedTensor.java