"V" not available as a design variable

Description

We currently cannot include the inflow velocity V among the design variables due to an incomplete variable setup routine.
This input variable must be included in the available set, which already includes mach, alpha, etc...
The bugfix will also require modifications to BaseClasses

Steps to reproduce issue

On a working runscript for ADflow, include 'V' among the design variables with:
ap.addDV('V') where ap is your AeroProblem instance.

Current behavior

The initialization terminates abruptly with the following message:

File "<path-to-baseclasses>/baseclasses/pyAero_problem.py", line 525, in addDV
    raise ValueError('%s is not a valid design variable' % key)
ValueError: V is not a valid design variable

Expected behavior

ADflow should run and evaluate the sensitivities, including those w.r.t. the inflow velocity

Code versions

• BaseClasses v1.2.2
• ADflow v2.2.1