Add an tutorial for distributed memory parallel simulation of heat equation

[MarkMa1990]

due to the lack tutorial of solving non-uniform isotropic heat equation (step-26) in distributed memory parallel computing, this pull request aims to studying a moving laser beam heating of a sample covered with absorptive material.

docs are included, but the images are stored inside the folder, it may be required to move.

[bangerth]

If I understand you right, the novelty of the program compared to step-26 is that it runs in parallel and that it has a moving right hand side. These are topics that are already covered in other tutorial programs -- just not the combination. Is this about right?

If so, I think the program would actually be a better fit in the code gallery. Have you thought about that?

[MarkMa1990]

Yes, exactly. In fact, this is a simplified version of my Ph.D studying which is coupled with nanoparticles' growth and physical-chemical process. In that case, the code are more complicated. That's why I tried here to make a simplification.

Okay, I will move to contribute code gallery. Thank you.

[bangerth]

Great, thanks for being willing to move it there. I think it will be a great fit for the code gallery!