Output deformation to CGNS format

[hlkline]

Being able to produce a deformed mesh in cgns rather than SU2 format would be useful for:

• transferring geometries resulting from optimization cases to other simulation tools (whether another fluid solver or for multiphysics simulations)
• editting meshes when the files used to generate the mesh have been lost or are unavailable (currently there is no way to load an su2 mesh into pointwise, for example).
• general compatibility with external tools: 3rd party mesh deformation & generation, CAD, etc.

Since we can already read CGNS format hopefully that won't be too burdensome.

[WallyMaier]

#674 I believe this resolves this issue

[MicK7]

Sorry, but this is not implemented yet. (#674 does not solve this issue)

[stale]

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