With the new filter in place, a CarbonPeriod will be treated like a DatePeriod object because it is inherited from DatePeriod.
But as the filter creates and returns a new DatePeriod it's not a copy anymore.

The first idea was to make the filter generic but I'm not sure if all objects which inherit from DatePeriod would have the same constructor.

The second idea was to add a specific filter but it needs to be before the DatePeriod filter.
As there is already a prependFilter but not a prependTypeFilter function I've created a PR.

Best
Alex