refactor: rename unit_constants to references

Addresses https://github.com/mpusz/units/pull/261#discussion_r596663522.

test/unit_test/static/quantity_test.cpp

@@ -40,9 +40,9 @@ namespace {
 using namespace units;
 namespace si = isq::si;
 using namespace si;
-using namespace unit_constants;
+using namespace references;
 
-constexpr auto cgs_cm = cgs::unit_constants::cm;
+constexpr auto cgs_cm = cgs::references::cm;
 
 
 //////////////////////////////