Feature Request: Improve How Viscous Drag is Linearized in HydroDyn

[jjonkman]

Is your feature request related to a problem? Please describe.
Viscous drag in HydroDyn is implemented via the relative form of Morison's equation. During linearization of HydroDyn, these drag terms are linearized directly via the central difference method, but this is not a robust method in the absence of sea current or forward speed of the substructure. When the sea current and/or forward speed are zero, the terms associated with the linearized drag will be dependent on the perturbation size used in the central difference method. (The linearization is reasonable when sea current or forward speed are nonzero).

Describe the solution you'd like
A more robust linearization of the viscous drag terms in HydroDyn is needed, e.g., Borgman linearization.

Additional context
This issue has been known for a while, but I just realized it has not been posted here.