Extract and visualize the surface mesh

[anne-glerum]

Extract the surface mesh from the volume mesh and output it to vtk for visualizing things like the heat flux on the free surface, as discussed with @bangerth

[anne-glerum]

@bangerth Out of curiosity, have you given this any more thought?

[anne-glerum]

After talking to @bangerth, it might be worth finishing these PRs:

• #4061
• #4054 The discussion in #4054 concerns the duplication of code, i.e., each volume mesh postprocessor will need to be copied for the corresponding surface mesh postprocessor. Instead of waiting on a restructuring of the visualization interface, we could merge the PRs in the mean time and remove them again after restructuring.

[bangerth]

@anne-glerum Tag me in these PRs once you think they're ready for review.