The remaining sections are handled in the dumux course
and the examples.

[license] create_cmakelists.py includes SPDX header

[license] add SPDX headers to Doxygen

[license] use CC-BY-4.0 for handbook documentation

[license] bulk license .md documentation with CC-BY-4.0

[license] use CC-BY-4.0 for example documentation

[license] Add SPDX headers for fmt

[license] Add SPDX headers for tinyxml2

[license] Add SPDX headers for doxygen-awesome

[license] Add SPDX headers to installexternal.py

[license] Add SPDX headers to doubleexp integration

[license] partial licensing of doxygen math filter