Split in subsets

• split loaded data in train, val, and test datasets
• check stations

changed milestone to %Pre-Processing

changed the description

changed time estimate to 1h

added 2h 22m of time spent at 2019-11-21

added Doing label

added 12m of time spent at 2019-11-25

added 1h 45m of time spent at 2019-11-26

more time needed because of #26 (closed)

marked the checklist item check stations as completed

marked the checklist item split loaded data in train, val, and test datasets as completed

mentioned in commit 150e5349

added merged in Develop label and removed Doing label

assigned to @leufen1

closed via merge request !17 (merged)

mentioned in commit 1f2c0182