Try upping the basis size further

We are well into a regime of diminishing returns, but we'd better start
by seeing if the easy thing works.  Besides, setting this to 7 trips the
step limit in _generating_ the algorithm!

src/core/include/units/magnitude.h

@@ -33,7 +33,7 @@ namespace units {
 namespace detail
 {
 // Higher numbers use fewer trial divisions, at the price of more storage space.
-using Factorizer = WheelFactorizer<4>;
+using Factorizer = WheelFactorizer<6>;
 } // namespace detail
 
 /**

test/unit_test/static/prime_test.cpp

@@ -50,6 +50,7 @@ static_assert(WheelFactorizer<2>::coprimes_in_first_wheel.size() == 2);
 static_assert(WheelFactorizer<3>::coprimes_in_first_wheel.size() == 8);
 static_assert(WheelFactorizer<4>::coprimes_in_first_wheel.size() == 48);
 static_assert(WheelFactorizer<5>::coprimes_in_first_wheel.size() == 480);
+static_assert(WheelFactorizer<6>::coprimes_in_first_wheel.size() == 5760);
 
 static_assert(WheelFactorizer<3>::coprimes_in_first_wheel[0] == 1);
 static_assert(WheelFactorizer<3>::coprimes_in_first_wheel[1] == 7);