Make TrackSummaryTool work with existing track summaries

As is stands right now, the Trk::TrackSummaryTool will completely forego creating a track summary for a track if one already exists, even if the existing one is only partially filled. This works well under the assumption that tracks are only given a summary by the Trk::TrackSummaryTool, but this is a model we're moving away from. Therefore, we need to modify the Trk::TrackSummaryTool not to skip, but to update and fill gaps in existing track summaries. This will prepare us for receiving tracks with summaries from, for example, track fitters. There is a possible performance pitfall here where we calculate values twice, but this does not occur in practice.