Improved wave-current modeling in SeaState

[luwang00]

Feature or improvement description
This PR adds improved modeling of combined wave and current conditions in SeaState. A new input WvCrntMod is added to the SeaState primary input file.

WvCrntMod value	Effects
0	Simple superposition where the current velocity and acceleration are added to those of the waves. The current has no direct impact on the waves. This option reproduces the behavior of previous OpenFAST versions.
1	Include Doppler effect on the waves by solving the wave dispersion relation corrected for current. The input wave spectrum/time series are assumed to be those directly measured in a region with the current and are not modified by SeaState. Wavelengths are corrected for Doppler effect.
2	Full interaction between waves and current with both Doppler effect and wave amplitude/spectrum scaling for wave-current interactions. The input wave spectrum/time series are assumed to be those measured in a region without current and rescaled based on current velocity/direction.

Options 1 and 2 impose additional restrictions on the waves and current.

WvCrntMod value	Restrictions
0	No additional restrictions
1	First-order long-crested waves without directional spreading
2	First-order long-crested waves without directional spreading; colinear (aligned or opposing) waves and current only; the wave amplitude/spectrum scaling implemented assumes deep-water conditions, although this is not checked

Note that strictly speaking, Options 1 and 2 are only valid for a uniform current; however, sheared current is allowed, and it is up to the user to ensure that the current profile near the surface down to a depth relevant to the waves is reasonably uniform. SeaState simply takes the current velocity at the still water level when computing the effects of the current on the waves.

The improved wave-current modeling can be used with either SeaState current or dynamic current from InflowWind when simulating marine hydrokinetic turbines. For the latter, wave-current interaction is based on the time-averaged current velocity at the still water level. For applicable WindType in InflowWind, users should ensure that the flow-field grid from InflowWind reaches the still water level.

WvCrntMod has no effect when WaveMod = 0 or 6, or when there is no current, either from SeaState (CurrMod = 0) or InflowWind.

Impacted areas of the software
SeaState, glue-codes, r-test, docs

Test results, if applicable
Updated all r-test SeaState input files with the new WvCrntMod input. No change to existing r-test results. Added two new r-tests for the SeaState module with WvCrntMod = 1 and 2.

To-Dos

• Update the user input documentation for SeaState
• Add new r-test

[jjonkman]

I only reviewed the documentation change and that looks good to me. I suggest uploading the Word-based implementation plan/theory document (which I previously reviewed) with this code change, unless you plan to publish that elsewhere.

[andrew-platt]

• Merge @luwang00's r-test branch with this PR (f/WvCrnt)

[andrew-platt]

• add Word based implementation plan to docs/OtherSupporting/SeaState/

[luwang00]

@andrew-platt Implementation plan added to docs/OtherSupporting/SeaState/