DerivationFrameworkCore: enable flake8 and make code compliant

...erivationFramework/DerivationFrameworkCore/CMakeLists.txt

...ramework/DerivationFrameworkCore/python/ContentHandler.py

...rivationFrameworkCore/python/DerivationFrameworkMaster.py

...ationFrameworkCore/python/DerivationFrameworkProdFlags.py

...work/DerivationFrameworkCore/python/LHE3WeightMetadata.py

...ramework/DerivationFrameworkCore/python/SlimmingHelper.py

...ramework/DerivationFrameworkCore/python/ThinningHelper.py

...ramework/DerivationFrameworkCore/python/WeightMetadata.py