Update docu and make the attribute optional.
Issue a warning in case it is set.

These are not used anywhere in the tests.

Fixes multiple copyright messages in the doxygen docu.

ag '\\file' -L --ignore=ThirdParty *Lib |xargs  sed -i 's/\(.*\)\\copyright/\1\\file\n\1\\copyright/'

Clang format run on near-the-header lines.

Happy New Year

Automatically with sed
sed -i 's/ogs_file_param_attr{process/ogs_file_param_attr{prj__processes__process/g'
and similar to the ogs_file_param_special and the ogs_file_param
doxygen commands.