Comparison of arbitrary number of metrics

[timgent]

Turns out DualDatasetMetricCheck is also helpful for comparing different metrics on a single dataset. So maybe the API/naming there could be improved.

The point of this ticket though is that it would also be helpful to compare an arbitrary number of metrics across one or many datasets.

This came up when looking at data where you can get the number of losses, wins, and the net. Each of those can be calculated in a metric, and 2 of them can be compared. However it isn't possible to compare all 3 of them. Could that be done? Possibly with composite metrics (i.e. Define a MetricDescriptor that is composed of other MetricDescriptors), possible with something else like taking a list of arguments rather than set number, or using type params to make things more flexible.