Pythia8_i Fixes for v.8.313: Implementation of the new "share/tunes" path, change of the naming of the bare LHA weights, update README with the latest AthGeneration release

1. Setting environmental variable PYTHIA8CMND so that Pythia can find the new share/tunes directory which includes new tune command cards.
2. Change from "AUX" to "EXTRA" prefix for the bare LHA Weights (not for analysis)
3. Update the README: change the instructions to make a sparse checkout of 'Pythia8_i' with the latest AthGeneration release, 23.6.45.

First change resolves the tune presets being missing during event generation. Before the set of the variable, it led to a "PYTHIA Error in Settings::readString: did not find file tunes/Monash2013-ee.cmnd" error that did not stop the generation but was present in the log files.

The second change is done so that downstream analysis tools (such as Rivet) automatically ignores the not_for_analysis weights. The name of the LHA bare weights was changed from "AUX_bare_not_for_analyses" to "EXTRA_bare_LHE_weight".

PhPy8 ttbar samples were generated after these changes are implemented. It was seen that the aforementioned error related to tune command cards disappeared from the log files. Additionally, MC_TTBAR routine was applied to the EVNT file to verify that the yoda files included the bare LHE weights under the name 'EXTRA_bare_LHE_weight".

Pythia Experts Team: @mshapiro @abuckley @gpanizzo @yiyu Added as watchers: @ewelina @ancuetog @yharris

added Generators label

This merge request affects 1 package:

• Generators/Pythia8_i

This merge request affects 3 files:

• Generators/Pythia8_i/Pythia8EnvironmentConfig.cmake
• Generators/Pythia8_i/README.md
• Generators/Pythia8_i/src/Pythia8_i.cxx

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CI Result SUCCESS (hash 8124eda8)

	Athena	AthGeneration
externals
cmake
make
tests

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AthGeneration: number of compilation errors 0, warnings 0
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Changes are fine and CI succeeds, approving for L1. -L1

added review-approved label and removed review-pending-level-1 label

merged

mentioned in commit 1a29b81e