Add .dtd back to Athena
Try to solve .dtd issues in ATLSWUPGR-146 which are currently causing all Run4 simulation to fail.
Adding a new package for this one file seems like overkill, but I couldn't think of another appropriate fit. Suggestions are welcome.
Longer-term we may be able to solve this a more elegant way, but in the meantime we should resolve this problem so that we can see if there is other fallout from the move to an external