v0.15.7

StartUpDG v0.15.7

Diff since v0.15.6

Merged pull requests:

• Write vtk (#81) (@Davknapp)
• Add quads to vtk writer (#83) (@Davknapp)
• CompatHelper: add new compat entry for WriteVTK at version 1, (keep existing compat) (#84) (@github-actions[bot])