Make CWL types constants for tab-completion

Copied from https://github.com/nlppln/nlppln/issues/31

> When adding inputs, it would be nice if there was a list of CWL types available in nlppln, so that I could use tab-completion when entering them, e.g.
> 
> with nlppln.WorkflowGenerator() as wf:
>   something = wf.add_input(something=nlppln.cwl_type. [TAB TAB TAB]
> 
> This way, I don't have to know all the CWL types by heart.