Now the API does not rely on `Grid` structure but lay a wrapper around
an existing memory region, which defines for example a diffusion grid.
All simulation steps are done in place.
The user has to make sure the memory existing throughout the lifetime of
a simulation grid.
Refactor testDiffusion.cpp and Diffusion.hpp to improve code readability and maintainability. Remove unnecessary exception throwing and replace with assert statements for invalid arguments.
