Bugfix: NDArary/NDView overflow in calculating number of elements (#322)

Fixed number of elements calculation that caused integer overflow. Now
return size_t instead of int.

src/NDView.test.cpp

@@ -8,8 +8,27 @@
 #include <vector>
 
 using aare::NDView;
+using aare::num_elements;
 using aare::Shape;
 
+TEST_CASE("Calculate size from a shape") {
+    Shape<3> shape{2, 3, 4};
+    REQUIRE(num_elements(shape) == 24);
+
+    Shape<2> shape2{5, 5};
+    REQUIRE(num_elements(shape2) == 25);
+
+    Shape<1> shape3{10};
+    REQUIRE(num_elements(shape3) == 10);
+
+    Shape<3> shape4{1000, 512, 1024};
+    REQUIRE(num_elements(shape4) == 524288000);
+
+    // 10GB which is more than INT_MAX bytes
+    Shape<3> shape5{10000, 1024, 1024};
+    REQUIRE(num_elements(shape5) == 10485760000);
+}
+
 TEST_CASE("Element reference 1D") {
     std::vector<int> vec;
     for (int i = 0; i != 10; ++i) {