make w2a initialization more accurate, especially for low resolutions

[mbkuhn]

Summary

For coarse mesh resolutions, the current initialization can end up applying wave velocities that are significantly above the interface, which are unrealistically high. This change makes things more careful in setting up the initial velocity based on where the liquid actually is.

Questions remain on whether this approach should be used for the forcing too, but that gets into some finite volume complexity.

Pull request type

Please check the type of change introduced:

• 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: