Revert "Try upping the basis size further"

This reverts commit 87073856a7.
It didn't fix the problem, and it caused some new ones.  We need a
different approach.

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<6>;
+using Factorizer = WheelFactorizer<4>;
 } // namespace detail
 
 /**