Fix heat flux postprocessor material_id processing

[AmishgaAlphonius]

Description of the problem

• In the previous implementation of the heat flux postprocessor, there was a confusion related to the material_id of a cell. As a cell's material_id is defined in the mesh, fluids moving from a cell to another did not have the right material_id when outputted. Futhermore, some dealii meshs have an intrinc way of numbering material_ids (e.g. colorized subdivided_hyper_rectangle) that were leading to inadequate comparison of the material_ids.

Description of the solution

• As of this PR, in the heat flux postprocessor, the material_id is only used to indentify cell material when solids are present. When simulating with fluids and solids, cells of the mesh in which the fluids lie should have a material_id of 0.
• For fluids-only simulations, at the moment, their heat fluxes will be output on the entire domain, as the current DataPostprocessor can only hold 1 dof_handler. During postprocessing, the user could clip the domain of interest of each fluid using the phase or filtered_phase fields outputted.

How Has This Been Tested?

• Visual inspection of outputted results for a:

• single fluid simulation,

• 1 fluid + 1 solid simulation, and

• 2 fluids simulaiton.

Future changes

• For multiple-fluids simulations, a DataPostprocessor handling multiple dof_handlers to allow postprocessing of quantities of interest of fluids in their respective subdomains will have to be added to the code.

[blaisb]

Nice fix, thank you :)!