BlockGSStep: Default value for BitVector

The third of four template arguments of the new BlockGSStep class is 'BitVector'. In the vast majority of the cases, this will be

Solvers::DefaultBitVector_t<Vector>

Please provide this as a default template parameter.

This may involve switching the order of the template parameters around[1], which is fine with me, even though it will break things.

[1] Unless one provides a default value for the LocalSolver template parameter, too, which I wouldn't mind.