Added regression tests for new SeaState and HydroDyn features

[luwang00]

The new regression tests covers:

• Wave stretching (WaveStMod)
• Constrained NewWave Model
• WaveMod=7 for user-defined wave frequency components
• MacCamy-Fuchs diffraction correction for strip-theory memebers with and without wave stretching
• Alternative robust calculation of nonlinear hydrostatic loads on strip-theory members (MHstLMod=2)
• Potential-flow wave excitation with the structure at the displaced position (ExctnDisp=1 and ExctnDisp=2)

Also includes small updates to the input file comments.

Related pull request to the OpenFAST repository updating reg_tests/CTestList.cmake: https://github.com/OpenFAST/openfast/pull/2031