External optional link to FLUKA 2025

[afedynitch]

By linking chromo with FLUKA binaries, it is possible to use chromo for running the event generator inside FLUKA.

Limitations:

• the random number generator remains that of FLUKA
• particle decay can't be customized by set_stable. Since chromo's DecayHandler is enabled by default, Pythia8 decay methods are used to decay the remaining short-lived particles
• All projectile and target nuclei are not yet supported, since some automation/interfacing with FLUKA's material functions are needed.
• Can be only compiled from source and won't be distributed via pypi any time soon. To enable FLUKA modify the pyproject.toml's enabled_models list.
• need tests

[afedynitch]

@jncots did you re-enable the failing tests?

[jncots]

@afedynitch, all tests are enabled and "ignore warning" is deleted. The core problem is solved. I change the implementation of "Root(Writer)._write_buffers(self)" that caused the Future warnings. The previous implementation has reliance on undocumented features of uproot when __setitem__ gets dictionary with empty string. When this occurs the internal mktree is initialized automatically.

The fix changes this automatic implicit initialization to explicit initialization of mktree container by passing "schema" of the data (branch_types in the code). As initialization is explicitly indicate that we use TTrees, the uproot warnings about default container which will change to RNTuples doesn't show up anymore. Those warnings was the original problem that failed tests.