operator equal for STensor33

Solver/STensor33.h

@@ -52,6 +52,11 @@ class STensor33 {
     for (int i = 0; i < 27; i++) res._val[i] += other._val[i];
     return res;
   }
+  STensor33& operator = (const STensor33 &other)
+  {
+    for (int i = 0; i < 27; i++) _val[i] = other._val[i];
+    return *this;
+  }
   STensor33& operator += (const STensor33 &other)
   {
     for (int i = 0; i < 27; i++) _val[i] += other._val[i];