time-series with ignore_year=true but spanning multiple years only preserves the last

[manuelma]

As per the title. It is not difficult for a user to miss that they have checked the ignore_year flag - but SpineInterface silently removes the data for any years before the last and that can easily lead to very hard-to-track errors.

We could issue a warning maybe even at DB API/Editor level to prevent the user from making this mistake. But the warning could also simply be at SpineInterface level.