Merge branch 'feature/more_file_readers' into 'master'

Add Vtk reader and Gmsh4 reader

See merge request !207
4 jobs for master in 45 minutes and 15 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #20259
dune-2.6

00:25:48

passed #20260
dune-2.7

00:34:05

passed #20261
dune-git

00:32:27

passed #20262
dune-git-full

00:41:57