Ising Model Temperature Parameter

I've added a temperature parameter in the Ising layers which can be either learned or set to a fixed value.

For now it only accepts a theano shared variable and it has no default value, but I'm not sure that's the right way to go. The thing is I think all layers need to share the same temperature (would the contrary make sense?), so I expect only one shared variable to be shared across layers. In that sense, it could  even be better to implement the temperature at a DBM-level instead of at a layer-level.

If the temperature can be set layer-wise, then I could add a sensible default and loosen the restrictions on what the layer expects as a temperature parameter. I could also implement the temperature in regular DBM layers (in __init__.py), it's pretty straightforward.