simple transforms

Removes the old transform classes and machinery to make simple transforms use history under the hood.
Changes the `add_transform` method to accept a string naming the simple transform and a window length (unless you pass returns).

simple transforms are now actual methods on the SIDData that do internal calls to history and apply the proper transforms.
While you're at it, can you add a doc string here? Not sure what it does / when it's used jsut from looking at it.