Random seeds for noise and tissue relaxation time generation

This PR adds the possibility to set the random seeds for various random events, enabling one to control the reproductibility of the scripts. Default behavior is still the same, that is to use a random seed based on the time of the computer.

I've changed all those I could find quickly, but there may be other random events that I'm not aware of.

As a (weird) added bonus, it seems like the diffusion amongst the phantom is now correctly generated (could most also probably be due to other upstream changes).