Remove ChannelFlow.density and replace with just incflo.density

[moprak-nrel]

Summary

ChannelFlow seems to define it's own density, and it is unclear why this needs to be a separate parameter from incflo.density. This PR consolidates those.

Pull request type

Please check the type of change introduced:

• [x] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes
• [x] Other (please describe): Consolidating density into one parameter for ChannelFlow

  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
  • [x] on CPU
• the regression tests
  • [ ] on GPU
  • [x] on CPU

Additional background

Issue Number:

[moprak-nrel]

This PR also fixes a potential bug with the wall function, previously this assumed that density is 1.0, and set nu = mu instead of nu = mu/rho.