Viscous walls with euler solver gives error when writing output files.

[bigfooted]

When setting up a problem with the Euler solver + isothermal walls (or heat flux walls), the solver apparently runs, but the volume output tries to write the skin friction, which does not exist.

https://www.cfd-online.com/Forums/su2/258285-problem-running-su2-euler-solver-windows.html

[pcarruscag]

I'll put a check