theta0 =0 -> tree force should be exact but isn't

lib/node.hpp

@@ -53,7 +53,7 @@ private:
     unsigned nLocalParticles_;
 
     double totalMass_;
-    const double theta0_ = 0.1;
+    const double theta0_ = 0.0;
     const double minNParticles_ = 32;
     const double maxDepth_ = 24;
     bool isLeaf_;