WDL workflow output definitions with functions

[mr-c]

https://github.com/broadinstitute/warp/blob/4dc972976cb0640e58321cd4746579a3fc0dee62/pipelines/broad/dna_seq/germline/variant_calling/VariantCalling.wdl#L205

  File "/home/michael/wdl2cwl/wdl2cwl/main.py", line 950, in get_cwl_task_outputs
    meta_name = item.info.expr.expr.name
AttributeError: 'Apply' object has no attribute 'expr'

The above exception was the direct cause of the following exception:
[…]
wdl2cwl.main.ConversionException: ../src/warp/pipelines/broad/dna_seq/germline/variant_calling/VariantCalling.wdl:205:23: 'Apply' object has no attribute 'expr'

[mr-c]

That particular example depends on implementing select_first from #161 (merged)