WIP enum example

[tobyhodges]

I've created an example of using enum syntax in a CWL wrapper to grep with the --directories={read|skip|recurse} option. Not sure if this is appropriate? @mr-c will probably be suspicious of my use of type: Directory...

I wanted to use a GNU tool that would be installed as standard, at least on linux/mac devices.

Any thoughts on a better example/how to improve this one?

[tobyhodges]

once accompanying lesson is written, this would address point 4 in #5

[tobyhodges]

actually, scrap this. I hadn't realised that grep returns non-zero exit status when it encounters a directory while in 'read' mode. That makes things too complicated.