Restricting usage of owner<T> to pointer types (#507)

* Restricting usage of owner<T> to pointer types

* Removing an additional type that was created for testing

* Added comment about the new constraint on owner

* Adding dereference operator to not_null

* Removing dereference operator changes for not-null

* Removing dereference operator changes for not-null

* Review comments

tests/owner_tests.cpp

@@ -20,6 +20,8 @@
 
 #include <functional>
 
+#include <memory>
+
 using namespace gsl;
 
 SUITE(owner_tests)
@@ -34,6 +36,16 @@ SUITE(owner_tests)
         CHECK(*p == 121);
         delete p;
     }
+
+    TEST(check_pointer_constraint)
+    {
+        #ifdef CONFIRM_COMPILATION_ERRORS
+        {
+            owner<int> integerTest = 10;
+            owner<std::shared_ptr<int>> sharedPtrTest(new int(10));
+        }
+        #endif
+    }
 }
 
 int main(int, const char* []) { return UnitTest::RunAllTests(); }