* src/plotypus/predictor.py: New module which will contains new
  predictor class `Baart`, which is not yet implemented.

Added `autocorrelation` to `plotypus.utils`, which will be utilized in
`Baart`.

Also made some minor touch-ups, such as removing unneeded functions from
`__all__` in `plotypus.preprocessing`, and added an unused `**params`
argument to `plotypus.preprocessing.Fourier.transform`, in order to
conform to `sklearn`.