[DF] Replace `dask_ml.wrappers.Incremental` with custom `Incremental` class

[sarahyurick]

As part of our initiative to move away from Dask-ML, we should replace the dask_ml.wrappers.Incremental class with our own custom class.

This implementation should be very similar to the ParallelPostFit implementation found in #832. I plan to work on Incremental after we merge ParallelPostFit.