Validate the format of the 'bins' parameter when splitting data by bins

Check type of the 'bins' parameter and its components in the wrapper
functions 'split.data.by.time' and 'split.data.by.bins'. Move
'split.data.by.time.or.bins' into a new category for internal helper
functions to discourage direct invocation.

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