Parallelize source site generation for fixed source calcs

src/source.cpp

@@ -328,6 +328,7 @@ void free_memory_source()
 void fill_source_bank_fixedsource()
 {
   if (settings::path_source.empty()) {
+    #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 + overall_generation()) *