nanostat: add check for quality scores

[phue]

Fixes #1717

• This comment contains a description of changes (with reason)
• CHANGELOG.md has been updated

Currently, the Nanostat module only checks if data was obtained from alignments, and then assumes that it contains quality scores.
With this change, it instead checks if the data actually contains quality scores, and skips plotting them if it's not the case.

[ewels]

Thanks!