[IDEA] Diffusion in a Sphere, Cylinder

[kookma]

First thank you very much for this interesting repo. It is useful for advanced numerical methods course in engineering.
I suggest few more examples.

1. Diffusion in sphere (1D problem)
2. Diffusion in a cylinder (1D problem)

Also, it would be great to add mass diffusion with linear reaction as example (slab, cylinder, sphere). I would be happy to provide you with analytical solution and checking the code/results for.

Also a sample nonlinear example would be great!