Simple maxout brick

Are you sure that we want to couple the max-taking part with the linear transform before it?

Perhaps it would be better to make maxout just another non-linearity, like Sigmoid and Tanh, and one would be able to alternate them in an MLP. The challenge is that unlike other non-linearities it has different input and output dimensions.

A possible solution: non-linearities can implement the `get_dim` interface and the MLP brick can query it to make its job of configuring linear transformations.