Enable Gamma Russian Roulette as ATLAS SteppingAction

[SeverinDiederichs]

This PR adds the gamma Russian Roulette for ATLAS and resolves #532.

The Russian Roulette is implemented as a SteppingAction: if active (-DADEPT_STEPPINGACTION=ATLAS), before a gamma is created, it will undergo RR. The settings for the energy threshold and probability are hardcoded for now, as the default parameters in Athena. One important difference: Athena checks for the placed volume prefix "LAr" while AdePT does this at the logical volume level. A safety check is added, and it does not trigger in Athena, in case a placed volume would be created without the LAr prefix, while the logical volume had one.

Notes:

• example1 was not using the weight in the energy deposition (integrationBenchmark was), this is fixed
• A new constructor for the track is needed, to assign the weight not directly from the parent pass it as a parameter

Performance results:

Run on the RTX4090

Config	Mean Real [s]	Std [s]	Mean wall [min]
Mono_Master	891.0	11.5	14.850
Mono_NoRR	909.9	3.0	15.165
Mono_GammaRR	860.3	11.6	14.338
Split_Master	898.0	8.6	14.967
Split_NoRR	904.6	2.0	15.077
Split_GammaRR	855.1	4.9	14.252
G4HepEm	1149.9	2.2	19.165

Validation:
The results in Athena with both monolithic and split kernels look good with the Russian Roulette turned on.

To do:

• update Readme and documentation to describe the new option
• do performance tests with Athena
• do physics validation in Athena

For an upcoming PR:

• Add CI test. The reason to not do it in this PR, is that it requires changing the drift test to use a different compilation flag. Doing in a separate PR will allow to show that the current test passes cleanly and then a separate PR will be made to change the CI test to include a case with Russian Roulette.

[phsft-bot]

Can one of the admins verify this patch?

[SeverinDiederichs]

/run-test

[agheata]

Thanks for this important addition. As discussed offline, the new photon roulette only preserves expectation for unit-weight parents, but this is how it is done in ATLAS so it has to match, propose a fix in Athena and only after update in AdePT. LGTM