align/resample sets flags to unflagged

unfortunately i approved the MR before i wrote the comment 🤦

The following discussion from !658 (merged) should be addressed:

• @palmb started a discussion:

in the current case (which was not changed by the MR !658 (merged)) all flags are explicitly set to UNFLAGGED. besides that i like this behavior, because of idea new data - new flags it might be unexpected to others. up to now this was shadowed by the early exit of shift, which i guess was quite often used by our pipelines/users. We might want to set NaN instead of UNFLAGGED, to make align be transparent (except for the meta), as it was in the shift-case before.