Replaced Time.now with a "Simulation Time" service (for simulation only)

Time is saved and freezed at start-up. Then only a PUT /simulation-time request can make it progress by one day up.
Mostly hard-coded, meant to be enhanced, here are some suggestions:
- One should be able to choose between simulation-time and "normal" time.
- One could need to choose the starting date, or even set the date instead of just requesting for one day up.
- One could need to specify the step (n days up instead of just one).
- One could need to request for the current date (instead of getting it while getting current problem).