(finish implementing) incremental-update of summaries

[jeremyh]

Currently there's no easy way to update cubedash's product summaries once created.

There is a max_age when the spatial table is updated, but not when generating the summaries themselves, so they are skipped if they exist at all (in the get_or_update() function).

We should extend the generate command to allow updating summaries older than a certain age.

(this was never needed previously in the NCI instance as it has a freshly-cloned database every day)

Reported by @dunkgray, as this affected DEA Africa.

[alexgleith]

Fixed in: #84

[jeremyh]

Reopening this — we only have a “force” option to force recreation of every summary, but I’d like to add real incremental updates.