[common] generalize valueRange check

dune/stuff/common/ranges.hh

@@ -188,7 +188,7 @@ sequence valueRange(const T start, const T end, const T increment = Epsilon<T>::
   // sadly, no overloaded version of std::abs is available for
   // unsigned long long, so we compute the absolute value of increment
   // ourselfs
-  T incrementAbs = (increment >= T() ? increment : -increment);
+  const T incrementAbs = std::is_unsigned<T>::value ? increment : std::abs(increment);
   sequence ret(typename sequence::size_type(((end > start) ? end - start : start - end) / incrementAbs), start);
   typename sequence::size_type i = 0;
   std::generate(std::begin(ret), std::end(ret), [&]() { return T(start + (increment * i++)); });