Update HeatFluxAux to use variable thermal conductivity

[mattfalcone1997]

Summary

This PR updates the HeatFluxAux by changing the input constant conductivity with an optional parameter diffusion_coefficient, which mirrors that in the HeatConduction kernel, with a default to the thermal_conductivity property set in the materials. The tests have been updated to reflect this change (by deleting the line specifying the thermal conductivity in the Auxkernel).

Related Issue

Resolves #23

Checklist

• Tests have been updated
• Verify comments/documentation updated to reflect changes.

[hsaunders1904]

This all looks great to me. It might be nice to have a test that checks the heat flux is actually changing over a varying k. It's a potentially non-trivial test to implement and I think this is a simple enough change that we could get a way without it. I'll leave it up to you!

[mattfalcone1997]

This all looks great to me. It might be nice to have a test that checks the heat flux is actually changing over a varying k. It's a potentially non-trivial test to implement and I think this is a simple enough change that we could get a way without it. I'll leave it up to you!

I think it would be best to have the test before we forget about it. I'll work it this week! Also, a good opportunity to work my way around the test system.

[hsaunders1904]

Looks great!