Named pipes should be used for inputs and outputs that both have the property `streamable`

In http://www.commonwl.org/v1.0/CommandLineTool.html#CommandOutputParameter (streamable):

> An implementation may use this flag to indicate whether it is valid to stream file contents using a named pipe

It would be nice if cwltool implemented this.

Start with the Common Workflow Language CommandOutputParameter specification section linked in the issue, then locate cwltool's handling of streamable inputs and outputs. Done means valid streamable files use named pipes for both input and output paths, with coverage for the relevant execution behavior.

Written by the indexing model from the issue text.