Make `InteractionTransformer` more useful

[ablaom]

If features are x, y, z, then this transformer only generates (with order=3) columns for x, y, z, xy, yz, xz, xyz, ie, no feature appears with degree higher than one, and there is no constant term. I suppose that is what "interaction" means?

But what about the common use case of polynomial regression on a single feature? You want 1, x, x^2, x^3, which this transformer won't give.

Perhaps we can add a parameter interaction=true which when false adds all the monomials up to specified order.

cc @olivierlabayle

[olivierlabayle]

Sounds good to me, should probably be a default to be fair.

If you are happy to rename, something like PolynomialTransformer might be more adequate with `interaction_only=true' the current behaviour.

[ablaom]

That's a good idea. We add a PolynomialTransformer with an `interaction_only` option (defaulting to `false`) and deprecate the `InteractionTransformer`, to be removed in the next breaking release.