`Tailfin_FreeYaw1DOF_PolarBased` airfoil has linear cl curve

[bjonkman]

When running the OpenFAST Tailfin_FreeYaw1DOF_PolarBased case, this warning appears:

FAST_InitializeAll:AD_Init:Init_AFIparams:AFI_Init:ReadAFfile:Airfoil data should go from -180 degrees to 180 degrees and the coefficients at the ends should be the same.

It looks like the airfoil file in that case has a linear cl curve instead of something periodic. See https://github.com/OpenFAST/r-test/blob/main/glue-codes/openfast/Tailfin_FreeYaw1DOF_PolarBased/Polar_2PiAlpha.dat

[andrew-platt]

@abhineet-gupta, can you look at this?

[andrew-platt]

Replaced this airfoil with the NACA-0018 with PR #137 (thanks @abhineet-gupta)

This test case is a toy problem, so any sensible looking airfoil table should work.