Split Metric into "Selector" and "Metric"

[lapplislazuli]

The current Metric has a lot of stuff in it, which are not about Metrics, but about selection. These things should be seperated.

Proposed Solution Split Metric.hs into Metrics.hs and Selection.hs

Possible Alternatives:

• Keep as is
• Split Selection into "Selection" and "Selection internals" just like Paths

Possible Problems: None, it's just effort, and maybe it doesn't make more sense after the split

Related Issues: None.