SMatrix: a generic fixed size D1 x D2 Matrix class.
    The class is template on the scalar type, on the matrix sizes:
    D1 = number of rows and D2 = number of columns
    amd on the representation storage type.
    By default the representation is MatRepStd<T,D1,D2> (standard D1xD2 of type T),
    but it can be of type MatRepSym<T,D> for symmetric matrices DxD, where the storage is only
    D*(D+1)/2.
    See \ref SMatrixDoc.
    Original author is Thorsten Glebe
    HERA-B Collaboration, MPI Heidelberg (Germany)
    @ingroup SMatrixSVector
    @authors T. Glebe, L. Moneta and J. Palacios
 SMatrix: column-wise storage