Add support for nested and total aggregation

For counts (e.g., numSkips, numScans) it is useful for the counts from all activities within a subgraph to be summarised, and to include all values from child queries.

I suggest NumTotalXXX could be used as a derived statistic, which should be possible for the jlib classes to summarise automatically.

For other stats (E.g., TimeLocalExecute) it is useful for the activity to include the sum of all the child-query/function execution times, but doesn't make so much sense to total the values all the way up the graph.

I suggest TimeNestedXXX could be used to represent this kind of summary.

This should allow each statistic kind to indicate how it is summarised

to know the summary for