fix: use positional arguments in hepmc3.GenEvent constructor

The kwargs are sensitive to local variable name changes in the C++ code, as of a recent pybind11 version.

benchmarks/clustering/scripts/gen_particles.py

@@ -21,7 +21,7 @@ PARTICLES = {
 
 
 def gen_event(p, theta, phi, pid, mass):
-    evt = hm.GenEvent(momentum_unit=hm.Units.MomentumUnit.GEV, length_unit=hm.Units.LengthUnit.MM)
+    evt = hm.GenEvent(hm.Units.MomentumUnit.GEV, hm.Units.LengthUnit.MM)
     # final state
     state = 1
     e0 = np.sqrt(p*p + mass*mass)