[CWL] ignore const

[mr-c]

From https://github.com/erasche/argparse2tool/blob/master/README.rst#limitations

const argument of add_argument(). All constants must be specified in job files.

My reading of https://docs.python.org/2/library/argparse.html#const is that const is an internal implementation detail -- I suspect that it can be safely ignored for the purpose of generating command line invocations.

• find real world usage of const
• test ignoring const
• update readme and perhaps add a comment in the code if this is indeed ignorable