Rework sliding window approach of 'split.data.activity.based'

Rework the algorithm to create sliding windows in activity-based splitting.
Instead of cutting off half a range many elements at the end before building
sliding windows (which creates a lot of edge cases), build sliding windows with
every element up to the last one. Then remove the last incomplete range.
The contents of the last incomplete range will be fully included in the second
last range and therefore redundant.
Sometimes the last incomplete range is a regular range. Previously the last
range always had to be a regular range. This means that removing the last
incomplete range requires updating the tests.

Additionally fix and improve documentation of splitting methods and fix
minor spelling bugs.

This works towards #239.

Signed-off-by: Maximilian Löffler <s8maloef@stud.uni-saarland.de>