Water-tutorial

**Description of the tutorial or documentation**
The script for water tutorial needs changes in terms of both time-steps and refinement steps.
At the moment the parameters suggested for the user aren't optimal and the original ones will never let this tutorial work.
Firstly, for the temperature of 263 K the time-step should be closer to 1 fs, since the molecular motions aren't as fast as they are at 303 K, for example.
Then parameters for simulation and refinement has to make the whole thing doable for longer than 0 refinement steps on a simple laptop.
Let's say that we add more than just "epsilon" as a parameter for refinement then we probably won't be able to complete a single step in a week. :-(

**Additional details**
I can modify this tutorial in coming days.