fix multiple errors in eta for stokes waves

[mbkuhn]

Summary

Following these notes https://www.caee.utexas.edu/prof/kinnas/ce358/oenotes/kinnas_stokes11.pdf based on this paper https://johndfenton.com/Papers/Fenton85d-A-fifth-order-Stokes-theory-for-steady-waves.pdf

4th-order term was calculated incorrectly due to a typo.

Much bigger deal -- all of the terms proportional to eps needed to be divided by k.

Pull request type

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

Checklist

The following is included:

• new unit-test(s)
• new regression test(s)
• documentation for new capability

This PR was tested by running:

• the unit tests
  • on GPU
  • on CPU
• the regression tests
  • on GPU
  • on CPU

Additional background

Issue Number:

[mpolimeno]

Nice find!

[mbkuhn]

Matteo is going to do a separate PR to introduce better structure into the code, which will lead to more unit tests. For now, this is a fix that we are confident in. Will introduce diffs to the ow_stokes reg test.