FANG event Generator

[itay-space]

This Pull request:

Adds the Focused Angular N-body event Generator (FANG) to the montecarlo directory.

Changes or fixes:

• New Feature: Implemented FANG, a Monte Carlo tool for efficient event generation in restricted (or full) Lorentz-Invariant Phase Space (LIPS).
• Unlike conventional approaches that always sample the full 4pi solid angle, FANG can directly generate events in which selected final-state particles are constrained to fixed directions or finite angular regions in the laboratory frame.
• Files Added:
  • Source code in montecarlo/fang/src and headers in montecarlo/fang/inc.
  • Tests in montecarlo/fang/test.
  • Tutorials in tutorials/evegen (runFANG.C, runParallelFANG.C).
• Authors: Arik Kreisel and Itay Horin.
• Reference: Horin, I., Kreisel, A. & Alon, O. Focused angular N-body event generator (FANG). J. High Energ. Phys. 2025, 137 (2025). https://doi.org/10.1007/JHEP12(2025)137

Checklist:

• tested changes locally (Test suite included in montecarlo/fang/test/testFANG.cxx)
• tutorials added