Patch for DD4Hep #862: do not change momentum in getParticleDirection

Isotropic DDG4 gun generator cannot be run with fixed energy. We rely on that for single particle production.

Upstream issue: https://github.com/AIDASoft/DD4hep/issues/862

No PR: not sure how they'll want to handle this upstream.

spack/packages/dd4hep/0001-do-not-change-momentum-in-getParticleDirection.patch

+diff --git a/DDG4/src/Geant4IsotropeGenerator.cpp b/DDG4/src/Geant4IsotropeGenerator.cpp
+index 54588aaf..f63b29b4 100644
+--- a/DDG4/src/Geant4IsotropeGenerator.cpp
++++ b/DDG4/src/Geant4IsotropeGenerator.cpp
+@@ -48,7 +48,6 @@ void Geant4IsotropeGenerator::getParticleDirectionUniform(int, ROOT::Math::XYZVe
+   double x3 = std::cos(theta);
+ 
+   direction.SetXYZ(x1,x2,x3);
+-  momentum = rnd.rndm()*momentum;
+ }
+ 
+ /// Particle distribution ~ cos(theta)
+@@ -63,7 +62,6 @@ void Geant4IsotropeGenerator::getParticleDirectionCosTheta(int, ROOT::Math::XYZV
+   double x3 = cos_theta;
+ 
+   direction.SetXYZ(x1,x2,x3);
+-  momentum = rnd.rndm()*momentum;
+ }
+ 
+ /// Particle distribution flat in eta (pseudo rapidity)
+@@ -84,7 +82,6 @@ void Geant4IsotropeGenerator::getParticleDirectionEta(int, ROOT::Math::XYZVector
+   double x3         = std::sinh(eta);
+   double r          = std::sqrt(1.0+x3*x3);
+   direction.SetXYZ(x1/r,x2/r,x3/r);
+-  momentum = rnd.rndm()*momentum;
+ }
+ 
+ /// e+e- --> ffbar particle distribution ~ 1 + cos^2(theta)
+@@ -110,7 +107,6 @@ void Geant4IsotropeGenerator::getParticleDirectionFFbar(int, ROOT::Math::XYZVect
+       double x2 = std::sin(theta)*std::sin(phi);
+       double x3 = std::cos(theta);
+       direction.SetXYZ(x1,x2,x3);
+-      momentum = rnd.rndm()*momentum;
+       return;
+     }
+   }