Change UserProp in Standard Linearization Output from Nodal to Collective/Independent

[jjonkman]

Is your feature request related to a problem? Please describe.
With the inclusion of user-defined airfoil aerodynamic property (UserProp from AeroDyn) as a standard linearization input with the merging of PR #612, OpenFAST is now better set-up to support controls design for active flow control of rotor blades. However, the implementation is per node per blade, so, the number of standard linearization inputs is large (NumBl*NumBlNds additional entries included in the input vector). While the inputs that are not needed in the linearized system can simply be ignored by the user, including a large number of standard inputs is computationally expensive and makes the linearization files large. This problem was highlighted recently in issue #738.

Describe the solution you'd like
The linearization of UserProp would be more useful if it was implemented with two modifications:

1. Have one input cover a range of stations along the blade (e.g., all aerodynamic nodes between 60-90% span have the same input)
2. Have both rotor-collective and blade-independent forms of the inputs, similar to how the blade-pitch control input is linearized.

This would reduce the number of standard inputs by NumBl*NumBlNds - 4.