Cleanup license headers

Unify user names in the license header authors list. Remove duplicate users in the authors list. Cleanupo of some other inor issues.

changed milestone to %APR21

resolved all threads

Dear @n.herrmann, @i.deppner, @f.uhlig, @p.-a.loizeau, @wielanek_AT_if.pw.edu.pl, @i.selyuzhenkov, @c.pauly, @v.friese, @v.singhal, @c.muentz, @m.deveaux, @s.lebedev, @praisig, @karpushkin_AT_inr.ru, @se.gorbunov, @v.akishina, @d.smith, @andrey.lebedev,

you have been identified as code owner of at least one file which was changed with this merge request.

Please check the changes and approve them or request changes.

added CodeOwners label

merged