As discussed in the last dev meeting, this deprecates the centered scheme for weakly coupled heat transfer, because it had a bug in the speed of sound, and because this made it much easier to adapt the rest of the heat solver to the scalar framework.

Bonus:

• Add SLOPE_LIMITER_HEAT option.
• OpenMP of the heat solver.
• Correct issues with Inlet and Outlet boundaries and gain farfield (not implemented before) for free.
• Regression for weakly coupled heat equation (primal was not covered before).
• PyCustom markers for heat solver.

ToDo:

• Bounded scalar mode.
• Docs.
• Fix sensitivity wrt inlet velocity.