Move channel flow wall model assert on body force to a better place

[moprak-nrel]

Summary

We require that the normal direction is 2 and that the body force in the 2 direction is 0 when using a wall model. However the constructor for the wall model gets initialized even when a wall model for the channel flow even when not using one. This PR moves the assert to the right place.

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

Additional background

This fixes the reg test channel_mol_mesh_map_z from failing.

Issue Number:

[marchdf]

Thank you!