Non-zero denominator contract added for ratio

src/include/units/ratio.h

@@ -29,6 +29,7 @@
 #include <type_traits>
 #include <tuple>
 #include <algorithm>
+#include <gsl/gsl_assert>
 
 namespace units {
 
@@ -50,6 +51,7 @@ struct ratio {
 
   explicit constexpr ratio(std::intmax_t n, std::intmax_t d = 1, std::intmax_t e = 0): num(n), den(d), exp(e)
   {
+    Expects(den != 0);
     detail::normalize(num, den, exp);
   }