Ability to reset database

The safety of this is of course, completely implementation-specific. But provided nothing is going to try to write to the database, all should be good.

Also, the database stack is retained all the way up the chain (moc retains parent retains PSC) so even if there is a leftover reference to the main MOC, and pending operations (in theory) should not crash if they are performed after the reset, but I think an error will be raised when trying to save to disk...