switch to python-package-template

this MR migrates the package to use the python-package-template template via cruft.

see https://codebase.helmholtz.cloud/hcdc/software-templates/python-package-template

this involves a migration to the reuse library which is why a lot of copyright-statements and license stuff has been changed.

in short:

• everything under docs/ is now licensed under CC-BY-4.0 with copyright by GFZ and Hereon
• everything under demessaging/ got copyright headers for GFZ, HZG and Hereon

ping @nils-brinckmann

this migration has been discussed (via mail) with Martin Hammitzsch and Christian Meeßen from GFZ