Proper definition of the abstract Machine template class

Created by: laurentes

For the major release 1.2.0, I would be in favour of clearly:

1. Defining what a machine is: To my mind, it is something that can be trained (like the term 'machine' in 'machine learning'), and that ouputs something given some input.
2. Updating the current abstract class API. To my mind, a machine should have:

• forward's methods
• load/save methods
• copy constructor, assignment operator, and comparison operators (==, != and is_similar_to)

Once done, we should update the 'machine' module accordingly. In addition, for each template, the generic Machine can be bound into python, which will help us to have a more consistent API.