[Rivet] Update DELPHI b fragmentation

[Markus Seidel]

Dear Rivet team,

would it be possible to add the b-fragmentation paper from DELPHI?
https://inspirehep.net/record/890503

I think it is just an update to DELPHI_2002_069_CONF_603, but the new 
version agrees better with the predictions for low x_B^weak.

Cheers,
Markus