More thorough tests for the filtering.py module

[niksirbi]

Is your feature request related to a problem? Please describe.
PR #116 introduced the filtering.py module and corresponding tests test_filtering.y which reach 100% coverage. However, due to time constraints we haven't implemented tests to cover all edge cases (e.g. currently only a single example dataset is tested).

Describe the solution you'd like
Test the filtering functions against a more complete set of sample data and thoroughly validate the expected behaviour.
As an example, we could try manually introducing series of consecutive NaNs with varying lengths in an existing dataset, and check that the correct amount of NaNs are removed/interpolated over.