Add static_assert to constructor of AxisAlignedCubeGeometry

Using AxisAlignedCubeGeometry with dim != coorddim and the constructor taking only two arguments leads to potentially wrong results.

The constructor's comment actually hints to this issue:

    /** \brief Constructor from a lower left and an upper right corner

        \note Only for dim==coorddim
     */
    AxisAlignedCubeGeometry(const Dune::FieldVector<ctype,coorddim> lower,
                            const Dune::FieldVector<ctype,coorddim> upper)

Since both dim and coorddim are known at compile time, a static_assert could prevent wrong usage. Alternatively, this constructor could be disabled by SFINAE.

Is there any reason why this is not done yet?