Apply DRY to fan speed 'should scale zero to one' functionality.

Some printers use 0.00-1.00 others use 0.0-256.0 as scale for their fans. This was (nearly) duplicated earlier in the PR.

done as part of CURA-6410

src/gcodeExport.cpp

@@ -1434,6 +1434,8 @@ void GCodeExport::writeFanCommand(double speed, std::optional<size_t> extruder)
 
 void GCodeExport::writeSpecificFanCommand(double speed, size_t fan_number)
 {
+
+
     const auto iterator = current_fans_speeds_.find(fan_number);
     const std::optional<double> current_fan_speed = (iterator != current_fans_speeds_.end()) ? std::optional<double>(iterator->second) : std::nullopt;
 
@@ -1464,16 +1466,17 @@ void GCodeExport::writeSpecificFanCommand(double speed, size_t fan_number)
     else
     {
         const bool should_scale_zero_to_one = Application::getInstance().current_slice_->scene.settings.get<bool>("machine_scale_fan_speed_zero_to_one");
+        const auto scale_zero_to_one_optional = [should_scale_zero_to_one](double value) -> PrecisionedDouble {
+            return { (should_scale_zero_to_one ? static_cast<uint8_t>(2) : static_cast<uint8_t>(1)), (should_scale_zero_to_one ? value : value * 255.0) / 100.0 };
+		};
         bool write_value = true;
         std::ostringstream new_value;
-        new_value << PrecisionedDouble{ (should_scale_zero_to_one ? static_cast<uint8_t>(2) : static_cast<uint8_t>(1)), (should_scale_zero_to_one ? speed : speed * 255) / 100 };
+        new_value << scale_zero_to_one_optional(speed);
         const std::string new_value_str = new_value.str();
         if (current_fan_speed.has_value())
         {
             std::ostringstream old_value;
-            old_value << PrecisionedDouble{ (should_scale_zero_to_one ? static_cast<uint8_t>(2) : static_cast<uint8_t>(1)),
-                                            (should_scale_zero_to_one ? current_fan_speed.value() : current_fan_speed.value() * 255) / 100 };
-
+            old_value << scale_zero_to_one_optional(current_fan_speed.value());
             write_value = new_value_str != old_value.str();
         }