QC: score-based output

[ivan-aksamentov]

We might want to extend the current QC algorithms to not only produce binary Yes/No output, but also provide more fine-grained results.

For example, for every rule we may produce a numeric score.

These scores can then be used to sort rows by severity of issues and to provide user with more fine-grained message text, upon which it will be easier to take an action.

It is preferable for scores for different rules to be numerically comparable for easier sorting and filtering. That means that more severe issues should have higher weights and produce higher "badness" (or lower "goodness") score. We may allow user to tweak these weights for their needs, to adjust desired QC strictness.