Flat Gas external generator

[jackal1-66]

Implemented new loopers external generator based on the previous one (still available): the number of loopers is now adapted to the time interval between one event and the other with a reference set through a parameter in the external gen. The reference loopers number is assigned to the calculated average time interval and a ratio is performed to adjust the loopers event by event. The loopers time is set via a Uniform distribution via a new generateEvent function.
The output from the generator can be found /alice/cern.ch/user/m/mgiacalo/selfjobs/LoopersFlatGasNewExtMean500-20250808-124127. The TPC clusters of this output were processed and they are available here /alice/cern.ch/user/m/mgiacalo/selfjobs/ProcessLoopersNewGenMean500-20250808-141601.
A flat average of ~10% of loopers clusters is achieved with the default configuration provided by this PR, as shown in the following time plot (in TPC cluster time units).

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[jackal1-66]

Merging. Test fails because of missing collisioncontext file. This generator is not supposed to work without it and it will be moved to AliceO2 in the future.