Implementation DataPrep class

This class prepares data to be used in neural networks. Especially the following steps can be performed

• interpolate: interpolate between data points by using xarray's interpolation method
• standardise: standardise data to mean=1 and std=1, or just centralise to mean=0
• make window history: to present the history (time steps before) for training/ testing; X
• make labels: create target vector for training/ testing; y
• remove Nans jointly from desired input and output, only keeps time steps where no NaNs are present in X AND y
• some other methods to ensure that the functions above are working properly

This class was originally created by @kleinert1 and is now transfered to this repository.

• implement class
  • interpolate
  • standardise
  • make window history
  • make labels
  • remove Nans
  • other
• write tests
• documentation