implement generator variations

A very preliminary implementation of the weights has been implemented, and only recently tested by @euslan (after a small bugfix in local, consisting in adding lhe as second argument in L298). Currently, we receive the following fatal exception:

----- Begin Fatal Exception 29-Mar-2023 11:27:41 CEST-----------------------
An exception of category 'BadToken' occurred while
   [0] Processing  Event run: 1 lumi: 2823 event: 32520780 stream: 0
   [1] Running path 'p'
   [2] Calling method for module Ntupliser/'ntupliser'
Exception Message:
A get using a EDGetToken with the C++ type 'LHEEventProduct' was made using a token with a value 1097297513 which is beyond the range used by this module.
 Please check that the variable is being initialized from a 'consumes' call from this module.
 You can not share EDGetToken values between modules.
----- End Fatal Exception -------------------------------------------------

To be followed up with GEN experts...