Consider changing io.ascii meta `comment_lines` to `comments`

The recommendation in APE6 for storing comments is a list called `comments`, which I think this is more natural than `comment_lines`. 

This would be an API change, but I suspect that until now (with #3222 in place) there are very few people that actually use `comment_lines`.  We could also simply add a line `meta['comment_lines'] = meta['comments']` at the end for at least one release of back-compatibility.

Comments @hamogu @mdmueller?