Implement Iterator

Design idea: "Iterates on data"

Iterator has next method to loop over a DataCollection. It returns a list of X and Y to use for an arbitry task. There could be multiple versions, e.g. a normal loop (for pre-/postprocessing) and a distributed loop (according to a batch size). The Iterator should meet all requiremnts to use in parallel for keras.

Jobs:

• has methods next to return X and Y return class object (that has get_X and get_Y)
• has an abstract class not required
• has inheritances that handle different kind of loops
  • normal loop (StandardIterator, connected with DataCollection from #142 (closed))
  • distributed loop (KerasIterator, requires DataCollection from #142 (closed) as input)