[ccomb]

Save named sets of parameter values on a database

A scenario is a name and the parameter values that make it, such as
"high salt" for salt_kg at 0.02. GET /db/{db}/scenarios lists them,
PUT /db/{db}/scenarios/{name} saves one, replacing what that name held,
and DELETE /db/{db}/scenarios/{name} drops it. To score one, a client
sends its values as the parameters of a scoring request, so the engine
never scores a scenario by name.

Scenarios are kept in a journal of their own, scenarios/journal.jsonl
in the database's upload directory, apart from the database's journal:
saving one changes nothing in the database, so it must not look like an
edit to the cache or to a replay. Nothing is held in memory; each listing,
save and drop reads that file.

Refused, with the reason: values a scoring request would refuse (an
unknown parameter, a formula, a process that is not there), a scenario
that sets nothing, a drop of a name that holds none, and any save on a
database the engine reads from its configuration, where the scenario
would be lost at the next restart. A scenario that an edit of the
database has since made unusable is listed with the problem. A copy of a
database takes its scenarios along.

The assistant has list_scenarios, save_scenario and drop_scenario,
and pyvolca scenarios(), save_scenario() and drop_scenario(). Wire
revision 75.