Merge pull request #1630 from smharper/omp_source_sampling

Sample source sites with shared memory parallelism

src/source.cpp

@@ -292,6 +292,7 @@ void initialize_source()
 
   } else {
     // Generation source sites from specified distribution in user input
+    #pragma omp parallel for
     for (int64_t i = 0; i < simulation::work_per_rank; ++i) {
       // initialize random number seed
       int64_t id = simulation::total_gen*settings::n_particles +