Merge branch 'comment-in-barriers' into 'master'

[bugfix] Define the comment field of barrier

The scheduler uses it, so we need to set it:

```
File ".../loopy/codegen/control.py", line 154, in generate_code_for_sched_index
    sched_item.kind, sched_item.comment)
AttributeError: comment
```

See merge request !23