[OmarDuran]

Add building blocks for mixed-MFD for thermal single-phase flow. This PR depends on #4139. The adaptive-MFD concept is applied uniformly to each elliptic component, namely Darcy’s law and Fourier’s law.

To provide a sense of validation the following snapshots represent a simple thermal convection cell documented in [1]. The critical Rayleigh number is 4π², so the onset of the instability occurs at Ra > 4π² .

The boundary conditions are:

• Energy: T = 320 K on z = 0 and T = 300 K on z = 100 m. Laterals are thermal insulated.
• Flow: All boundaries are impervious. Neumann problem is solvable on this setting because the density derivatives are non-zero.

Initial conditions are:
Hydrostatic pressure: p = 1.0e7 + 9810·(100 − z) Pa.
The conductive vertical profile plus the critical mode at 0.01 K amplitude: T = 320 − 0.2 z + 0.01·sin(πz/100)·cos(πx/100).

The following snapshots present the approximations at the final time of 2.0e10 s (634 years) and Ra = 50. The top panel shows the MFD approximation on a simplex mesh, for which the gravity and gradient terms are consistent. The middle panel shows the TPFA solution on a hexahedral mesh, where the discretization is fully consistent. Finally, the bottom panel shows the TPFA solution on a simplex mesh. In this case, the discretization is clearly not consistent failing to capture the physics of the convection cell.

Reference:
[1] E. R. Lapwood, "Convection of a fluid in a porous medium", Math. Proc. Cambridge Philos. Soc. 44 (1948), 508–521, https://doi.org/10.1017/S030500410002452X.