check doctests using pytest

The current code already has many doctests in the functions docstrings. pytest has convenient methods to execute and check doctests:

python3 -m pytest --doctest-modules

unfortunately, all of the currently available tests fail. The tests (and code) should be checked and fixed. Future commits should be checked automatically e.g. using Gitlab CI. A potential .gitlab-ci.yml could look like:

pytest:
  image: "python:3.10"
  stage: test
  before_script:
    - pip install -r requirements.txt
    - pip install pytest
  script:
    - pytest --doctest-modules --junitxml=report.xml
  artifacts:
    when: always
    reports:
      junit: report.xml

☝ this has not yet been tested, and currently lacks the requirements.txt. Maybe a better option would be to install the moist_thermodynamics package instead of using requirements.txt, maybe @m300575 knows how to do that the most elegant way :-)