Add started_when and stopped_when date-time fields to Simulation data model

[Baguage]

Moved from redmine, ticket 6363
These fields are needed by the VecNet CI to know when a simulation started, and how long it took to run. They should be assigned automatically when the simulation's status is updated with a PATCH request:

• When status becomes STARTED_SCRIPT, set the started_when field to the current time
• When status becomes SCRIPT_DONE or OUTPUT_ERROR or SCRIPT_ERROR, set the stopped_when to the current time