employing more mathemtaically correct ratio_gcd calc (for common ratio)

really finds the maximum common ration as opposed to previous algo which
simplified on the exp part of the ratio by using std::min
most of new code credit to Conor Williams
discussion and additional doc here:
https://github.com/mpusz/units/issues/62#issuecomment-588152833
test case was 1yd + 1in = 37in => added as a test
commenting out unusued ratio_add and its tests
if to be reintroduced, should also use the new gcd routines
additonal change was required to check in `safe_divisible` concept
den=1 is not sufficient anymore. reusing new gcd routines
moved ratio nomalize and new gcd routines into new, separate bits/ratio_maths.h
this resolves #62

test/unit_test/runtime/fmt_units_test.cpp

@@ -170,4 +170,9 @@ TEST_CASE("fmt::format on synthesized unit symbols", "[text][fmt]")
   {
     CHECK(fmt::format("{}", 1q_Npm) == "1 N/m");
   }
+
+  SECTION("addition with common ratio")
+  {
+    CHECK(fmt::format("{}", 1q_in + 1q_yd) == "37 in");
+  }
 }