Add a bias in the multiplicative wrapper's xWm calculation
Add a bias in the multiplicative wrapper's xWm calculation

This way Marian's implementation matches Amun's. Also the math is a bit more
general than in the previous implementation.

emjotde/amunn,amunmt/marian,amunmt/marian,emjotde/Marian,emjotde/amunmt,emjotde/amunn,marian-nmt/marian-train,marian-nmt/marian-train,emjotde/amunmt,marian-nmt/marian-train,emjotde/amunmt,marian-nmt/marian-train,marian-nmt/marian-train,emjotde/amunn,emjotde/amunn,emjotde/Marian,amunmt/marian,emjotde/amunmt