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

Move fmt to Dumux namespace

Closes #1150 (closed)

See merge request !3099 (merged)

(cherry picked from commit ffa6d36f)

dc5d7d03 [fmt] move implementation to namespace Dumux 27942de6 [test] Use Fmt in cornerpoint test