Short-circuit S_H if wanted

src/action.hpp

@@ -162,6 +162,9 @@ inline Scalar S_H(const array::ArrayView<const Mask, Shape0>& mask,
 
     Scalar S{0.0};
 
+    if (alpha == 0)
+        return S;
+
     for (int i = 0; i < nx-1; ++i) {
         for (int j = 0; j < ny-1; ++j) {
             if (mask(i,j) == Mask::VACUUM)