[bugfix] avoid subscript operator on JacobianTransposed

A geometry's JacobianTransposed implementation may be any type modelling a linear mapping, i.e., implementing the mv-type methods. Some grids (e.g., SPGrid) only implement this interface for performance reasons. However, we require that a JacobianTransposed can be assigned to a FieldMatrix.

This patch fixes "checkCodim1Mapping" such that it will cast the JacobianTranposed down to a FieldMatrix before accessing the subscript operator (although the same result could be achieved using "mv").