kiji-express: EXP-394. Provide groupByQualifier function for Seq[FlowCell[_]]

issue: https://jira.kiji.org/browse/EXP-394
review: https://review.kiji.org/r/1547
commit: https://github.com/kijiproject/kiji-express/commit/f6f87ebdfc5943f6528108950ecd5c9c5b3c7f84