Training error method

[scriptandcompile]

We might consider encapsulating the concept of a 'trainer' into the neural network just as we encapsulated the idea of a 'activation function' object into the neural layer.

this would leave training and error propogation as the job of the trainer and polymorphic and not something the neural network itself handles directly. This should clean up the issue of the name / enum switch thing while also offering the chance to expand into new training types in the future.

[afshinm]

Seems like a good idea. It makes NeuralNetwork easier to understand. My +1.