Add a learning trait

[sgreenbury]

Add trait that should be implemented by the learning model included in the board (global state) so that it can be generically included for different learning algorithms. For example:

pub trait Learning {
    fn init(&mut self);
    fn step(&mut self);
    fn loss(&self) -> f32;
}

pub struct Board <M: Learning> {
    ...,
    pub model: M
    ...,
}