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This adds the Distribution class, which will replace the
TargetDistribution typedef in Model.  It provides some
convenient functionality, such as += and max-finding.

None of the uses of TargetDistribution are updated here.

Change-Id: I0e86d9dbaddcd5421cbc65e73213329603346733
Reviewed-on: https://chromium-review.googlesource.com/c/1329881
Commit-Queue: Frank Liberato <liberato@chromium.org>
Reviewed-by: Fredrik Hubinette <hubbe@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611569}