For explicit diffusion operations, use designated state of the variable

[mbkuhn]

Summary

compute_diff_term has different input states, but only uses the new variable. This doesn't fit with fixed point operations. The PR makes compute_diff_term actually use the supplied state, and it modifies the boundary condition calls to fit that.

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