Make BlockGSStep default constructible

A typical construction of the new BlockGSStep class in my code is

Solvers::BlockGSStep<DisplacementMatrix,DisplacementVectorType, Solvers::DefaultBitVector_t<DisplacementVectorType>,  decltype(Solvers::BlockGS::LocalSolvers::direct())> step(Solvers::BlockGS::LocalSolvers::direct());

Note how I need to specify the local solver twice; that's pretty ugly. The constructor should be able to default-construct the local solver automatically.