Moved vec.m under matlab/missing/vec, and add it into the path only under...

Moved vec.m under matlab/missing/vec, and add it into the path only under MATLAB (vec exists under Octave)


git-svn-id: https://www.dynare.org/svn/dynare/trunk@2967 ac1d8469-bf42-47a9-8791-bf33cf982152