HydroDyn: AxVnCOff units missing

[RBergua]

Current versions of OpenFAST allow to include the suction-side only viscous drag formulation.

However, when looking at the input files (e.g., from r-test), the cut-off frequency appears as diminsionless:

I understand that the units are in reality (Hz). For reference, the documentation (Readthedocs: https://openfast.readthedocs.io/en/main/source/user/hydrodyn/input_files.html#axial-coefficients) shows the proper units.

I guess it would be good to fix the units missing for AxVnCOff in future releases.

[andrew-platt]

Completed with #3117