Add Archimedes submodule, with nested submodules

# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Changes to be committed:
#	modified:   .gitmodules
#	new file:   Archimedes
#
# Submodule changes to be committed:
#
# * Archimedes 0000000...17d04bb (29):
#   > Merge pull request #13 from github/better-boxing
#
