Add a new trait for fitting a transformer

[AtheMathmo]

As discussed in #166 .

We would like to add a new trait to ensure that a Transformer can only be used from a valid state. This means adding a trait like:

pub trait TransformFitter<U, T: Transformer> {
    fn fit(self, inputs: U) -> T;
}

And we will make it so that Transformers which require fitting before use cannot be instantiated except via this trait.