Simplify usage of heating-local variable

[aprilnovak]

OpenMC writes tally results into an auxiliary variable, which Aurora is currently:

1. Converting from an auxvariable to a user object
2. Converting from a user object to a function
3. Converting from a function to a material
4. Applying a material-based heat source

Steps 1-3 above could be replaced by just using the CoupledForce kernel directly. Image below provides more of a graphical depiction of the current transfers, which can be simplified.