Add tests for splitting into equally-sized windows (#49 and PR #140)

[clhunsen]

Description

With the final fix for issue #49 in PR #140, we definitely need tests for splitting data and networks based on a given amount of time windows (see also #140 (comment)).

Motivation

Tests are good! 😁

Ideas for the Implementation

We have three functions to test:

• split.data.time.based,
• split.network.time.based, and
• split.networks.time.based.

A test for the also affected function split.get.bins.time.based has been added in commit 8d80fa9 already.