Debug pulse based pad

Hi @amohammadi

Again, I had to do some modifications to my pulse-based PAD code ... I modified the handling of landmarks detector failing, since it does not raise an error anymore, but returns None.

Anyway, everything's green so I guess that you can merge this.

BTW, let me know if it annoys you to be assigned for each change I'm making ... I understood that it was a good practice not to auto-merge to master ... But since it may happen quite often in the near future, I don't mind doing it ;)