helper/expanded: provide a `stopnames` argument to limit expansion
For all matching series expression whose `name` argument is in
`stopnames`, expansion is skipped.
A use case will be some later chirurgy on the relevant expressions.
test/constant: exhibit an error related to time-travelling `today`
If we accept naive revision dates we probably should make sure
they get tz-aware, or we blow up.
Because of some troubled clients (xl ?) we will have to be forgiving.