Add cwl checker

[tom-tan]

This request adds a syntax checker for Common Workflow Language (CWL) that describes analysis workflows and tools.
Its syntax is defined as Semantic Annotations for Linked Avro Data (SALAD).
I added a syntax checker for CWL using schema-salad-tool (link to pip repository), which is a validator of SALAD format.

To test the CWL checker, I copied the schema files for CWL from the repository of schema salad, which are licensed under Apache-2.
I guess it is harmless to include these files to this repository for this purpose because Apache-2 and GPL-3 are compatible.

[CLAassistant]

All committers have signed the CLA.

[fmdkdd]

Seeing as there is only one expected error, do we really need to include all these files for testing? In other words, can you build a minimal test that triggers the same error?

[tom-tan]

Unfortunately, we can not reduce files in schema/ because they are referred by schema/CommonWorkflowLanguage.yml.

IMO it is difficult to remove other schema files.
If we modify the schema to remove the dependency, the test checks the message for modified CWL and it is not what we want to test.

[tom-tan]

ping.
Is it ready to merge or should we need a workaround not to use the schemas?

[fmdkdd]

LGTM; just a minor change to the Vagrantfile

[fmdkdd]

This needs a --user flag.

[fmdkdd]

@tom-tan I just needed to find some time to run the test locally. Thank you for adding one, by the way. This is a good PR all around.

Could you also add a line to CHANGES.rst under New syntax checkers? And squash your commits when you are done, then we can merge.

[tom-tan]

Done!

[fmdkdd]

Great! Nice work 👏 , and sorry for the delay.

[tom-tan]

Thank you for merging!