By David Alire Garcia MEXICO CITY (Reuters) -An extensive cache of Aztec ritual offerings found underneath downtown Mexico City, off the steps of what would have been the empire’s holiest shrine, provides new insight into pre-Hispanic religious rites and political propaganda. Sealed in stone boxes five centuries ago at the foot of the temple, the contents of one box found in the exact center of what was a ceremonial circular stage has shattered records for the number of sea offerings from both the Pacific Ocean and off Mexico’s Gulf Coast, including more than 165 once-bright-red starfish and u…