Aggregate host names in the JobAggregate when aggregating across hosts

[lars-t-hansen]

When jobs span hosts we want the host names to be aggregated in the JobAggregate, for subsequent analysis and display.

[lars-t-hansen]

It is possible (#24 (comment)) that we don't want to do this naively, though, but may instead want each job to have some kind of joint aggregated data but also a per-host breakdown of some of the data. So don't do this until we have figured that out.

[lars-t-hansen]

Also, aggregating the hosts is sort of annoyingly expensive, requiring a hash set or similar structure for disambiguation and an array for the result, with strings probably allocated for all the elements of the array unless we optimize with a string pool. At the moment we only do this for display, which is fine, but if we were to do it for many more records it would be good to consider the costs.

[lars-t-hansen]

It's OK for now to do this when we print, and not sooner. Ditto for the GPU sets.