Implement under-relaxation for the tight-coupling iterative solver

[luwang00]

This PR is not ready to be merged

Feature or improvement description
This PR adds under-relaxation for the tight-coupling iterative solver to address occasional solution divergence. Using an under-relaxation factor less than 1 can potentially improve the stability of numerically challenging models. It can also improve simulation time by avoiding oscillatory convergence and reducing the number of times the Jacobian needs to be updated.

Currently, a simple constant under-relaxation factor based on user input is implemented. Setting it to 1 removes under-relaxation and recovers the previous behavior. In the future, a default value such as 0.8 can be implemented. Alternatively, automatic under-relaxation based on the change in residual between successive iterations is also possible.

Impacted areas of the software
Glue-code

Test results, if applicable
r-test input files need to be updated with the new under-relaxation factor input. Results should not change if the under-relaxation factor is set to 1.

• r-test branch merging required

[jjonkman]

Dear @luwang00,

Thanks for this addition. Should it be that the relaxation factor reduces with each iteration of the nonlinear solve, and after convergence, the factor gets reset back to unity for the next time step? This is what we did within the nonlinear solve for the old quasi-static mooring module of FAST; I believe MAP++ does something similar. Perhaps with this method, the additional user input is not needed.

Best regards,