groups of metrics

[yegor256]

There are too many metrics already in the repository, it's hard to read the final report and dataset. Let's introduce "groups". Every metric, when it's generated by a script in the metrics/, may print it this way:

foo 999 This is a good metric
abc 999 [X1] This is a good metric

Here, foo doesn't belong to any group, while abc belongs to the X1 group. Then, in report.tex, we should list them by the groups, instead of a single long list.