Refactor metrics

[roym899]

There are some issues with the current metrics implementation.

Some thoughts:

• single estimate metrics / dataset metrics
• dataset metrics are typically defined by a single-estimate metric and aggregation function
• some metrics are composed of other metrics
• for some metrics bigger is better, for other smaller is better
• support swiping single-estimate metric parameters (like a threshold) and automatic plotting
• symmetry handling varies per-metric

Currently most of this is supported for the standard metrics, but it's quite messy.