Consistently declare size_type in preconditioners derived from PreconditionRelaxation.

[bangerth]

The base class declares a 'size_type' typedef, but only some of the derived classes
import it. Do so consistently, and do it by inheriting the type instead of
re-inventing the wheel.

Prompted by running into this static_assert

template <typename MatrixType>
template <class VectorType>
inline void
PreconditionJacobi<MatrixType>::vmult(VectorType &      dst,
                                      const VectorType &src) const
{
  static_assert(
    std::is_same<typename PreconditionJacobi<MatrixType>::size_type,
                 typename VectorType::size_type>::value,
    "PreconditionJacobi and VectorType must have the same size_type.");

...and having to figure out whether that type actually exists.

@juleoc02 -- FYI.

/rebuild

[drwells]

The intel check is having some trouble (not just here but in other PRs). I'm going to go ahead and merge since its probably fine.