wip: don't lock command mutex for each mass/spring in FillGraph()

include/physics.hpp

@@ -83,7 +83,7 @@ public:
   std::unordered_map<std::pair<State, State>, int> state_springs;
 
 public:
-  MassSpringSystem() {
+  MassSpringSystem() : threads(std::thread::hardware_concurrency() - 1) {
 #ifndef BARNES_HUT
     last_build = REPULSION_GRID_REFRESH;
     std::cout << "Using uniform grid repulsion force calculation." << std::endl;
@@ -188,6 +188,10 @@ public:
   auto AddSpringCmd(const State &a, const State &b) -> void;
 
   auto ClearCmd() -> void;
+
+  auto AddMassSpringsCmd(const std::unordered_set<State> &masses,
+                         const std::vector<std::pair<State, State>> &springs)
+      -> void;
 };
 
 // https://en.cppreference.com/w/cpp/utility/variant/visit