Merge branch 'fix/fmt-for-use-with-opm' into 'master'

Move fmt to Dumux namespace

Closes #1150

See merge request !3099

(cherry picked from commit ffa6d36f)

dc5d7d03 [fmt] move implementation to namespace Dumux
27942de6 [test] Use Fmt in cornerpoint test
5 jobs for !3108 with cherry-pick-ffa6d36f-2
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