select_first() implemented as WDL.

[DailyDreaming]

Description of WDL function to be added:

## X select_first(Array[X?])

Given an array of optional values, `select_first` will select the first defined value and return it. Note that this is a runtime check and requires that at least one defined value will exist: if no defined value is found when select_first is evaluated, the workflow will fail.https://github.com/openwdl/wdl/blob/main/versions/development/SPEC.md#x-select_firstarrayx

• [x] Add the following WDL function to https://github.com/DataBiosphere/toil/blob/master/src/toil/wdl/wdl_functions.py
  • Implemented here: https://github.com/DataBiosphere/toil/blob/master/src/toil/wdl/wdl_functions.py#L457
• [ ] Add a test that runs a wdl/json with toil and then checks against cromwell to verify that toil and cromwell have the same behavior See: https://github.com/DailyDreaming/wdl-conformance-tests/tree/master/tests/standard_library

For example:

version 1.0
workflow SelectFirst {
  input {
    Int? maybe_five = 5
    Int? maybe_four_but_is_not = None
    Int? maybe_three = 3
  }
  Int five = select_first([maybe_five, maybe_four_but_is_not, maybe_three]) # This evaluates to 5
  Int five = select_first([maybe_four_but_is_not, maybe_five, maybe_three]) # This also evaluates to 5
}

┆Issue is synchronized with this Jira Story ┆epic: WDL Standard Library Implementation ┆friendlyId: TOIL-650