public void reset() {
        for (int i = 0; i < bucketToColumn.length; ++i) {
            bucketToColumn[i] = 0;
        }
        numColumns = 1;
    }
Sets the number of columns to distribute to to 1, and resets the content of the internal bucket-to-column map so
that it all buckets point to that single column.
document/src/main/java/com/yahoo/document/BucketDistribution.java