Ability to merge headers during history compaction

[cdata]

As of #661 we have the ability to make a compact history out of a linear, contiguous set of sphere versions. However, the initial implementation discards all in the intermediate versions that make up the compacted version. We should add some kind of facility to make it possible to preserve headers (either mechanically or as some kind of filter) so that the final compacted version loses as little information from the original history as possible.

Another consideration to keep in mind is that a history that is compacted may lose information about original authorship of parts of the history, so it may be worth while to enable aggregating values at the step where we merge headers.