Merge branch 'develop' into 'release_v1.4.0'

.gitlab-ci.yml

HPC_setup/requirements_HDFML_additionals.txt

HPC_setup/requirements_JUWELS_additionals.txt

conftest.py

docs/_source/_plots/datahistogram.png

docs/_source/_plots/periodogram.png

docs/_source/defaults.rst

mlair/configuration/defaults.py

mlair/data_handler/abstract_data_handler.py

mlair/data_handler/bootstraps.py

mlair/data_handler/data_handler_kz_filter.py

mlair/data_handler/data_handler_mixed_sampling.py

mlair/data_handler/data_handler_single_station.py

mlair/data_handler/data_handler_with_filter.py

mlair/data_handler/default_data_handler.py

mlair/data_handler/iterator.py

mlair/helpers/datastore.py

mlair/helpers/filter.py

mlair/helpers/helpers.py

mlair/helpers/join.py