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.
I think that leaving the implicit truth value here is okay because if `initial_values={}` then this should still be avoided.