Fix and test for missing count division (#329)

Author: HDembinski

include/boost/histogram/detail/atomic_number.hpp

@@ -48,6 +48,12 @@ struct atomic_number : std::atomic<T> {
     return *this;
   }
 
+  // not thread-safe
+  atomic_number& operator/=(const T& x) noexcept {
+    this->store(this->load() / x);
+    return *this;
+  }
+
 private:
   // for integral types
   template <class U = T>

test/accumulators_count_test.cpp

@@ -48,6 +48,16 @@ void run_tests() {
     c_t two(2);
     auto six = two * 3;
     BOOST_TEST_EQ(six, static_cast<T>(6));
+    six *= 2;
+    BOOST_TEST_EQ(six, static_cast<T>(12));
+  }
+
+  {
+    c_t six(6);
+    auto two = six / 3;
+    BOOST_TEST_EQ(two, static_cast<T>(2));
+    two /= 2;
+    BOOST_TEST_EQ(two, static_cast<T>(1));
   }
 }