Add `outputEval` to WorkflowOutputParameter

[mr-c]

• Add outputEval to WorkflowOutputParameter
• in WorkflowOutputParameter.outputEval, CWL Expressions / CWL Parameter references would also gain a outputs object that contains all the pre-outputEval values, just like inputs is available in WorkflowStepInput
• implement in cwltool
• conformance tests galore

See #19 for motivation