hits_from_df forces event numbers to be int32

[gonzaponte]

The implementation of this function forces the event number to be a 32-bit signed integer. This creates a problem when processing backgrounds for NEXT100, where event numbers need to be represented as 64-bit integers.
I do not remember the reason for this choice and the comment in the code doesn't explain it fully.
We need to assess whether there is a real need for this restriction or if it's just a ghost from the past.
Initial checks by @martinperezmaneiro indicate that it is no longer necessary.