Merge remote-tracking branch 'gh/master' into docs

.github/workflows/run-tests.yml

.gitignore

.travis.yml

CHANGELOG.md

README.md

examples/.gitignore

examples/kspace-test-2d.py

postpic/_compat/functions.py

postpic/_field_calc.py

postpic/datahandling.py

postpic/datareader/__init__.py

postpic/datareader/datareader.py

postpic/datareader/epochsdf.py

postpic/datareader/openPMDh5.py

postpic/helper.py

postpic/helper_fft.py

postpic/io/image.py

postpic/io/vtk.py

postpic/particles/_routines.py

postpic/particles/particles.py