units: Keep power integral

When we compute powers for units with numerical results on, we may end
up with decimal powers in units for no reason.

Spotted by displaying the result of an `EvalEq` with `FIX 2`.

Fixes: #1086

Signed-off-by: Christophe de Dinechin <christophe@dinechin.org>

src/expression.cc

@@ -1260,6 +1260,7 @@ algebraic_p expression::simplify_products() const
 
     // Save auto-simplify and set it, and evaluate units normally
     settings::SaveAutoSimplify sas(true);
+    settings::SaveNumericalResults snr(false);
     save<bool> smode(unit::mode, false);
     save<bool> sexpr(unit::factoring, true);
 

src/functions.cc

@@ -205,6 +205,7 @@ algebraic_p function::evaluate(algebraic_r xr, id op, ops_t ops)
             uint divisor = 2 + (op == ID_cbrt);
             algebraic_g exponent = +fraction::make(integer::make(1),
                                                    integer::make(divisor));
+            settings::SaveNumericalResults snr(false);
             uexpr = pow(uexpr, exponent);
             return unit::make(value, uexpr);
         }