Add first file selector

Former-commit-id: d68d8379
Former-commit-id: 41f43806

steps/selectfirstfile.cwl

+class: ExpressionTool
+cwlVersion: v1.0
+id: select_first_file
+inputs:
+  - id: inputs
+    type: File[]
+    doc: input files
+outputs:
+  - id: output
+    type: File
+
+expression: |
+  ${
+    return {'output': inputs.inputs[0]}
+  }
+label: FileSelector
+
+requirements:
+  - class: InlineJavascriptRequirement