class AliAnalysisTaskPileup: public AliAnalysisTaskSE

             Implementation of class AliAnalysisTaskPileup

 The class re-weights the number of analysed events with the correction
 factor to account for the event pileup.

 The final idea will be to include all of the methods for the pileup
 calculation.

 For the moment, the implemented method is based on a work
 by L. Aphecetche.
 The probability of having at least 1 collision is given by:
 MB L0 trigger / max collision rate
 The max collision rate is given by:
    number of bunches X revolution frequency ~ CBEAMB L0 trigger
 Assuming a poissonian distribution of the collision, with:
    mu = mean number of collisions
 the probability of having 0 collisions is:
    exp{-mu} = 1 - L0b_MB / max collision rate
 Since the MB trigger is measuring the probability of having at least
 1 collisions, the number should be corrected by the factor:
    CF = mu / ( 1 - exp{-mu} )
 The class weights the number of events with this correction factor

 CAVEATS:
 - so far the class needs to access the OCDB
   (hopefully it will be changed in the future)
   Hence the following libraries need to be included (in addition to
   the standard analysis ones):
   libRAWDatabase.so libCDB.so libSTEER.so libPWGHFbase.so