Multiplicative integration

[MartinuzziFrancesco]

As described here, multiplicative integration modifies the recurrent equation from

$$h_{t} = \sigma(W \cdot x_t + U \cdot h_{t-1} + b)$$

to

$$h_{t} = \sigma(W \cdot x_t \odot U \cdot h_{t-1} + b)$$

This is applicable to virtually every cell which uses the recurrent equation (which is all of them). It would be nice to find a way to give the user a choice for each cell which version to use (:normal or :multiplicative_integration I guess), but in order to do so we would need to abstract a lot out of the current implementation