[dune-fem] Add typedef to #include PetscLinearOperator

[Agnese, Marco]

Hello,
could you please add the public typedef

typedef Mat MatrixType;

in the class Dune::Fem::PetscLinearOperator? 
It would be more consistent whit the other operators implementation.
Otherwise it is necessary to implement workarounds when using this type of operators.

Thank you very much,
Regards,
Marco.