filtering raw data in comparer fails

When slicing a comparer in time (either using start/stop or specific time=) for the purpose of calculating statistics over a sub-period, we also try to slice raw_data.

https://github.com/DHI/modelskill/blob/c50518cecdea0ccf1fe7239848728e593b8af7b3/src/modelskill/comparison/_comparison.py#L862-L875

If model results have been interpolated to the obs, the timesteps in raw data and matched data does not align. This can easily make sel(time=) fail since we have no raw_data on this time step.

Solution:
Try to slice raw_data, otherwhise set None.