Instead of raising an error on missing weight id the index could be used

https://github.com/scikit-hep/pylhe/blob/c5503c2f6fdcc3e8219233d8936c65e6d1173dca/src/pylhe/__init__.py#L271

eg. by setting
```
wg_id = index
```

Origin: https://github.com/scikit-hep/pylhe/pull/220/files/45dff93321a13ab64d3a87d40358567af4dd97ce#r1465950808