[WriteBAMTransform](https://github.com/googlegenomics/dataflow-java/blob/master/src/main/java/com/google/cloud/genomics/dataflow/writers/bam/WriteBAMTransform.java) used by [ShardedBAMWriting](https://github.com/googlegenomics/dataflow-java/blob/master/src/main/java/com/google/cloud/genomics/dataflow/pipelines/ShardedBAMWriting.java) assumes that it receives reads in order. Add a group by and a sort operation so that input reads can be read from multiple BAM shards in parallel.
WriteBAMTransform used by ShardedBAMWriting assumes that it receives reads in order.
Add a group by and a sort operation so that input reads can be read from multiple BAM shards in parallel.