Unique ID for simulation events

[mlincett]

In the discussion of #7 , @tianluyuan raised the concern about run_id and event_id not being unique for simulated events in the input file.

It is to be noted that the current i3_to_json.py functionality does not fully support the case of multiple events with same run_id and event_id. It is then out of scope of PR #7 to address this issue.

We currently still need a unique (run_id, event_id) for SkyDriver as well.

So either:

• the user makes sure that run_id and event_id are unique and properly defined in the input data;
• skywriter must find a way to map each simulated event to a unique run_id and event_id combination;
• SkyDriver implements an unique id that is not based on (run_id, event_id), but this becomes increasingly inconvenient for the user.