Making the two cube representations accessible to use + various fixes

[jjerphan]

Main changes:

• predict_data renamed to testing_data_released and predict_data/original to testing_data_released/test_data to follow submission guideline
• discretization.py: constructed classes (RelativeCubeRepresentation and AbsoluteCubeRepresentation) to make the two cube representations accessible:
  • it is the class job to construct appropriate cube now
  • ExamplesIterator now take such a CubeRepresentation
  • fixed some bugs
  • documented and tidied code
• capitalized global settings : global variables are now in CAPS to better identify them
• updated dependencies
• added tests