(Velocity-dependent) Dispersion for tpfa
Constant dispersion tensor are implemented in !2921 (merged).
Velocity-dependency could be realized by using a cell velocity based scheme and harmonic averaging of the dispersion tensor on the face. But this extends the flux stencil of a single flux to the element stencil required for the velocity reconstruction.
Follow-up from #320 (closed) and !2921 (merged).