Adds license headers

This merge request adds license headers according to the reuse specification.

Please note and double check:

• I committed on your behalf so that the commit history is not "polluted"
• I added GPL-3.0 -only because we recommend this instead of the -or-later variant (please notify me in case I should change thisThis merge requests add license headers according to the reuse specification.

Please note and double check:

• I committed on your behalf so that the commit history is not "polluted"
• I added GPL-3.0 -only because we recommend this instead of the -or-later variant (please notify me in case I should change this)
• files have, depending on the file type, different licenses:
  • all code is now GPL-3.0-only
  • trivial files such as config files are CC0-1.0 (public domain)
  • data/images (csv,png,vtk,..) are CC-BY-4.0 (you may contact GFZ Data Services and ask for a suitable data license)
• files where headers are not supported now have a filename.ending.license file that specifies their license. To avoid this, they could instead be summed up in a reuse.toml (see documentation)
• I added a note at the bottom of the README
• the repository is now reuse compliant:

$ reuse lint
# SUMMARY

* Bad licenses: 0
* Deprecated licenses: 0
* Licenses without file extension: 0
* Missing licenses: 0
* Unused licenses: 0
* Used licenses: GPL-3.0-only, CC0-1.0, CC-BY-4.0
* Read errors: 0
* Files with copyright information: 47 / 47
* Files with license information: 47 / 47

Congratulations! Your project is compliant with version 3.3 of the REUSE Specification :-)