Update code

include/testlib.hpp

@@ -8,7 +8,8 @@
  *  @brief A small test library.
  *
  *  @details This a small testing library that is tailored for the needs of the
- * project.
+ *  project. Anything that is in the details namespace should not be used 
+ *  directly, or else it might cause undefined behavior if not used correctly.
  *
  *  @bug No known bugs
  * */
@@ -90,7 +91,7 @@ template <class T,
           class = typename std::enable_if<std::is_arithmetic<T>::value>::type>
 static bool close_to(T a, T b, double tol = 1e-8)
 {
-    return std::abs(a - b) < tol;
+    return std::fabs(a - b) < tol;
 }
 
 /** @brief Test if two armadillo matrices/vectors are equal.