[CWL] implement custom types and custom actions using strings

[mr-c]

As a workaround for
https://github.com/erasche/argparse2tool/blob/master/README.rst#limitations

Custom types and custom actions are not supported.

I would suggest that a string is a safe fallback.

• find real world example
• test this
• explain in the readme that custom types and custom actions are implemented using CWL's type: string