v1.9.1

• Change singularity invocation from singularity run to singularity exec (#628 @adthrasher)
• WDL version development:
  • Make the input: keyword optional in calls (openwdl/wdl#524)
  • miniwdl check: reduce spurious warnings on env task inputs used as environment variables instead of WDL interpolations (openwdl/wdl#504)