Add vars wrappers for numerical fluxes

[charleskawczynski]

Description

This PR:

• Add vars wrappers for numerical fluxes
• Adds a WrapVars type, to indicate when balance law kernels are wrapped in Vars

• [ ] Code follows the style guidelines OR N/A.
• [ ] Unit tests are included OR N/A.
• [ ] Code is exercised in an integration test OR N/A.
• [ ] Documentation has been added/updated OR N/A.

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[charleskawczynski]

Looks like this is finally ready. It adds a bit of code, but it simplifies the kernels a bit. Would it be better to change _loop! to _loop_bcs!?

[charleskawczynski]

Bump! cc @simonbyrne @jkozdon. This may make grepping a bit easier. I'm now regretting the _arr! suffix a bit.

[jkozdon]

@charleskawczynski can you close this? Seems pretty stale.