use masking of velocity diffusion based on terrain cells

[mbkuhn]

Summary

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:

[hgopalan]

One concern is if this can cause issues with the pressure solve?

[mbkuhn]

It should only affect the viscous solve, not the pressure solves.