Patankar exponential scheme

Exponential scheme utilizing the exact solution for the one-dimensional steady advection-diffusion problem for evaluating face fluxes using the finite volume method.

* Correction: in the video I said to multiply by 0.5 instead of dividing by 2 because division is more expensive. That's generally true, but in binary, the special case of halving or doubling is fast as it can be done with a simple bit shift.