Fix comment, remove unnecessary cast.
Only call the intrinsic version for 3x3 sym matrices (6x6 was broken for a while because of that).
Add accessors for the plexes and plex vectors used in the test.
Copy in SMatrix.
New test t4 -- compare results of Matriplex and SMatrix calculations to make sure we don't do anything silly there.
Ignore t4 executables.