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.
OK, I guess you have to do that here because it relies on information we don't have available during `__init__`? It's very clever, just wondering if not a bit too clever.