The block algorithms are now able to work on specific level

Src/GroupTree/Core/FGroupSeqAlgorithm.hpp

Src/GroupTree/Core/FGroupTaskAlgorithm.hpp

Src/GroupTree/Core/FGroupTaskDepAlgorithm.hpp

Src/GroupTree/Core/FGroupTaskStarpuAlgorithm.hpp

Src/GroupTree/Core/FGroupTaskStarpuMpiAlgorithm.hpp