Feature/23 tests filters transforms

This PR adds tests for our pre-defined filters and transforms.

Tests are written using pytest, are put into the directory of the filter or transform, and are automatically detected and executed in our CI pipeline if the files use the naming test_*.py or *_test.py. I used the names test_filter.py for filters andtest_transform.py for transforms.

I added an empty file, called __init__.py into each filter/transform directory to turn them into Python modules. This helps with isolating the code of different filters/transforms in a global namespace, as used by pytest.

I will further describe the testing approach and guidelines in our docs in the next few days.

Fixes #23

DataCater / datacater

Feature/23 tests filters transforms #40