Add new routine and variables to init in the ADI_c_binding interface

[andrew-platt]

Ready to merge

Feature or improvement description

1. Add routine to get the disk average velocity ADI_C_GetDiskAvgVel
2. Add WrVTK_DT to initialization to set the timestep between VTK writes. The ADI c-bindings library interface has not had a method setting a timestep between visualization VTK file writes. This results in a massive number of files if the timestep is small.
3. Add OutVTKdir to the initialization so that the VTK output directory can be specified. This can be an absolute path, or a path relative to the ADI input files. If this variable is empty, vtk-ADI is used (previous behavior).
4. Correction to the way disk average velocity was calculated in AeroDisk. This was noticed when adding the calculations to ADI-c and corrected in accordance with IEA-15MW-240 Free Decay Tests #2352.
5. Removal of redundant adilib in CMakeLists.txt
6. Renaming of internal variable in the AeroDyn_Driver_Subs.f90 file for better clarity and consistency with other codes

Related issue, if one exists
None. This was by request of an external user.

Impacted areas of the software
This changes the interface to the ADI c-bindings library.

Test results, if applicable
The test results do not change as the

[andrew-platt]

The input file for the py_ad_5MW_OC4Semi_WSt_WavesWN case was updated, so merge r-test as well.