Tutorials (SU2 versions 8.1.0 "Harrier")

Tutorial files associated with the SU2 v8.1.0 "Harrier" releases.

What's Changed

• Addition of preferential diffusion model for flamelet solver by @EvertBunschoten in #48
• [WIP] Fix symmetry by @bigfooted in #49
• Feature solid solid cht by @EvertBunschoten in #52
• Feature solid solid cht by @EvertBunschoten in #53
• Solid-solid CHT: screen output iteration by @EvertBunschoten in #54
• Develop by @pcarruscag in #55

Full Changelog: v8.0.0...v8.1.0