Use strong typed enum for MergeType

source/Lib/CommonLib/ContextModelling.cpp

source/Lib/CommonLib/DeblockingFilter.cpp

source/Lib/CommonLib/InterPrediction.cpp

source/Lib/CommonLib/TypeDef.h

source/Lib/CommonLib/Unit.cpp

source/Lib/CommonLib/UnitTools.cpp

source/Lib/CommonLib/dtrace_blockstatistics.cpp

source/Lib/DecoderLib/DecCu.cpp

source/Lib/EncoderLib/EncCu.cpp

source/Lib/EncoderLib/InterSearch.cpp