Remove the option DIFFUS that is meant to remove the diffusion

This issue was created automatically from an original CUE issue, opened by Agnes Leroy. Further discussion may take place here.

---

This option was originally used for optimisation or to make unit tests but should not be used in applications.
It has become quite dangerous to use it since the diffusion step may also treat the source terms, so using DIFFUS=NO may remove also the source terms.
To remove diffusion from a simulation the best is to set a zero diffusion coefficient.
Thus, we propose to remove the DIFFUS option, in order to simplify the code and to make the available options more secure for the users.