Make CWL types constants for tab-completion

[jvdzwaan]

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.