Without it you would get an error about multiplication being ambiguous
   in this case.
 - Added an overload to catch expressions of the form diag_matrix*regular_matrix*diag_matrix
   and turn them into a form which is slightly more numerically stable in some cases.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403932