The method might have been badly named, but I separated input space initialization in two method calls in order to allow BoltzmannIsingVisible to be in charge of its own Boltzmann parameters.

Since parameters in the Boltzmann space are initialized according to some parameters of layers above/below, there is a dependency that's impossible to satisfy when `set_input_space` is called sequentially on all layers.

I chose to factor out initialization that depended on parameter initialization into a separate method, and since the change was made in order to allow initialization of parameters that were used at sampling time, I named that method  `set_sampling_space`.

I'm very open to suggestions for a better name, though.