YAN-424 - CWL output to stdout

[james-strauss-uwa]

Make the "dlg cwl" command output a zip file to stdout when no output file is specified by the user.

The main change is in tool_commands.py, where the cwl command now uses _open_o() with opts.output to send output to stdout.

Other changes are:

• modified input parameters to create_workflow() and create_command_line_tool(). Previously, these methods wrote the intermediate workflow and step files to disk, before combining the files into a zip file for output. Now the contents of the workflow and steps are held in strings, used to create io.BytesIO objects, and the zip file is built in memory.
• added a common.s2b() method, since the io.BytesIO() method requires "byte-like" input in Python 3. I'd be interested in feedback on this approach.

[coveralls]

Coverage decreased (-0.09%) to 75.418% when pulling a33c237 on yan-424 into 4ce7018 on master.

[rtobar]

Looks great! My only suggestion is around passing the final file object as a parameter to create_workflow instead of accumulating in memory first; that way the writing is done directly to the final "target".

[rtobar]

Looks even better now, thanks for the great work :)