Example Zprime.sin triggers a bug

[Juergen Reuter]

The example Zprime.sin triggers

| Generating 100000 weighted, unpolarized events ...
| 
| |===================================================================|
| | % complete | events generated | events remaining | time remaining
| |===================================================================|
|      0.0               0             100000              ???
******************************************************************************
******************************************************************************
*** WHIZARD BUG: Event: write raw: particle set is undefined
******************************************************************************
******************************************************************************
| There were no errors and    1 warning(s).
WHIZARD run aborted.

Per definition, this _is_ a WHIZARD bug.

[Juergen Reuter]

Actually, it is the ?unweighted = false flag that triggers this bug.

[kilian]

No idea, but I'll look at it.

[kilian]

OK, this is caused by events which did not pass the cuts.

We might store these as events with weight zero, but actually the particle set is not even filled. (It wouldn't matter, of course.) These events should probably not count for n_events when generating weighted events, just be dropped silently.

[kilian]

r5335: weighted events are dropped if they do not contain a valid particle set.