Automatic add a fit_transform method to new transformers

[mandel]

It could be nice if when we create a new operator with a fit and a transform method, the functionoperators.make_operator automatically adds a fit_transform method with the following implementation (it is already the case for get_params):

def fit_transform(self, X, y=None):
    scaler = self.fit(X, y)
    return scaler.transform(X)

[mandel]

Closed by PR #1192