We claim the ornamental design for a cremation urn, as shown and described.
FIG. 1 is a front side plan view of a cremation urn, the rear side view having the same ornamental view;
FIG. 2 is a right side plan thereof, the left side having the same ornamental view;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a top side exploded view thereof; and,
FIG. 6 is a front, right side and bottom perspective view thereof, the back and left side being a mirror image.
The broken lines showing optional decorative elements on the cremation urn are for the purposes of showing environment and form no part of the claimed design.